{
  "index": "1993-B-6",
  "type": "NT",
  "tag": [
    "NT",
    "ALG",
    "COMB"
  ],
  "difficulty": "",
  "question": "Let $S$ be a set of three, not necessarily distinct, positive integers.\nShow that one can transform $S$ into a set containing 0 by a finite\nnumber of applications of the following rule: Select two of the three\nintegers, say $x$ and $y$, where $x \\leq y$ and replace them with $2x$ and $y-x$.\n\n\\end{itemize}\n\\end{document}",
  "solution": "Solution 1 (attributed to Garth Payne). It suffices to show that \\( (a, b, c) \\) with \\( 0<a \\leq b \\leq c \\) can be transformed into \\( \\left(b^{\\prime}, a^{\\prime}, c^{\\prime}\\right) \\), where \\( a^{\\prime} \\) is the remainder when \\( b \\) is divided by \\( a \\). (Hence any set with no zeros can be transformed into a set with a smaller element.)\n\nLet \\( b=q a+r \\) with \\( r<a \\); let \\( q=q_{0}+2 q_{1}+4 q_{2}+\\cdots+2^{k} q_{k} \\) be the binary representation of \\( q \\) (so \\( q_{i}=0 \\) or 1 , and \\( q_{k}=1 \\) ). Define \\( g_{0}(d, e, f)=(2 d, e, f-d) \\) and \\( g_{1}(d, e, f)=(2 d, e-d, f) \\). Then\n\\[\ng_{q_{k}}\\left(\\cdots\\left(g_{q_{1}}\\left(g_{q_{0}}(a, b, c)\\right)\\right) \\cdots\\right)=\\left(b^{\\prime}, r, c^{\\prime}\\right)\n\\]\ndescribes a sequence of legal moves.\nSolution 2. Let \\( a, b, c \\) be the elements of \\( S \\). We prove the result by strong induction on \\( a+b+c \\), a quantity that is preserved by applications of the rule. For the sake of obtaining a contradiction, assume that \\( S \\) cannot be transformed to a triple containing 0 .\n\nFirst we reduce to the case that exactly one of \\( a, b, c \\) is odd. If two are odd, apply the rule with those two, and then none are odd. If none are odd, divide all numbers by 2 and apply the inductive hypothesis. If three are odd, apply the rule once, and exactly one is odd. Once exactly one is odd, this will remain so.\n\nWithout loss of generality, \\( a \\) is odd and \\( b \\) and \\( c \\) are even. Let \\( 2^{n} \\) be the highest power of 2 dividing \\( b+c \\). We will describe a series of moves leaving us with \\( a^{\\prime}, b^{\\prime} \\), \\( c^{\\prime} \\), with \\( a^{\\prime} \\) odd and \\( 2^{n+1} \\) dividing \\( b^{\\prime}+c^{\\prime} \\). Repeating such a series of moves results in a triple \\( a^{\\prime \\prime}, b^{\\prime \\prime} \\), \\( c^{\\prime \\prime} \\) with \\( b^{\\prime \\prime}+c^{\\prime \\prime} \\) divisible by \\( 2^{m} \\) where \\( 2^{m}>a+b+c \\). Then since \\( b^{\\prime \\prime}, c^{\\prime \\prime}>0 \\), we have \\( b^{\\prime \\prime}+c^{\\prime \\prime} \\geq 2^{m}>a+b+c=a^{\\prime \\prime}+b^{\\prime \\prime}+c^{\\prime \\prime} \\), contradicting \\( a^{\\prime \\prime}>0 \\).\n\nWe first apply a series of moves so that \\( b \\) and \\( c \\) are divisible by different powers of 2 , and the one divisible by the smaller power of \\( 2(b \\), say \\( ) \\) is also smaller. If \\( b \\) and \\( c \\) have the same number of factors of 2 , then applying the rule to those two will yield both divisible by a higher power of 2 , or one will have fewer factors of 2 than the other. Since \\( b+c \\) is constant here, after a finite number of applications of the rule, \\( b \\) and \\( c \\) will not have the same number of factors of 2 . Also, possibly after some additional moves (on \\( b \\) and \\( c \\) ), the one of \\( b \\) and \\( c \\) divisible by the smaller power of 2 is also smaller.\n\nNow if \\( a>b \\), then apply the rule to \\( (a, b) \\); \\( a \\) remains odd, \\( b \\) is doubled, and \\( b+c \\) is divisible by a higher power of 2 as desired.\n\nOn the other hand, if \\( a<b \\), apply the rule first to \\( (a, b) \\), and then to \\( (b-a, c) \\). (Note that \\( c>b>b-a \\).) The result is the triple \\( (2 a, 2 b-2 a, c-b+a) \\). Now the odd number is \\( c-b+a \\), and the sum of the even numbers is \\( 2 b \\), which has one more factor of 2 than \\( b+c \\).\n\nSolution 3 (Dylan Thurston). As in Solution 2, we assume there is some triple \\( a, b, c \\) that cannot be transformed to a triple containing 0 , and that exactly one of \\( a, b, c \\) is odd. We give a recipe showing that if two of the triples are divisible by \\( 2^{n} \\), then we can reach a state where two are divisible by \\( 2^{n+1} \\); since the sum of the triple is constant, this eventually gives a contradiction.\n\nThe result is trivial if both of the multiples of \\( 2^{n} \\) are already divisible by \\( 2^{n+1} \\), or if neither are (just apply the rule once to that pair). So assume one is a multiple of \\( 2^{n+1} \\), and the other is not, so the triple is (after renaming) \\( (a, b, c) \\equiv\\left(0,2^{n}, x\\right)\\left(\\bmod 2^{n+1}\\right) \\) (where \\( x \\) is odd).\n\nWe can assume \\( a>c \\). (Otherwise, apply the rule to ( \\( a, c \\) ) repeatedly until this is so.) Then apply the rule to \\( (a, c) \\), giving the triple \\( \\left(a^{\\prime}, b^{\\prime}, c^{\\prime}\\right) \\equiv\\left(-x, 2^{n}, 2 x\\right)\\left(\\bmod 2^{n+1}\\right) \\).\n\nIf \\( a^{\\prime}<b^{\\prime} \\), then apply the rule to \\( \\left(a^{\\prime}, b^{\\prime}\\right) \\), giving \\( \\left(-2 x, 2^{n}+x, 2 x\\right)\\left(\\bmod 2^{n+1}\\right) \\). Repeated applications of the rule to \\( (2 x,-2 x) \\) eventually produce \\( (0,0)\\left(\\bmod 2^{n+1}\\right) \\) (regardless of which is bigger at each stage).\n\nIf on the other hand \\( a^{\\prime}>b^{\\prime} \\), apply the rule to \\( \\left(a^{\\prime}, b^{\\prime}\\right) \\) giving \\( \\left(2^{n}-x, 0,2 x\\right) \\) \\( \\left(\\bmod 2^{n+1}\\right) \\). Apply the rule repeatedly to the 0 and \\( 2^{n}-x \\) terms until the 0 is bigger, and then apply it once more to get \\( \\left(-2 x, 2^{n}+x, 2 x\\right)\\left(\\bmod 2^{n+1}\\right) \\). Again apply the rule repeatedly to \\( (2 x,-2 x) \\) to eventually produce \\( (0,0)\\left(\\bmod 2^{n+1}\\right) \\).",
  "vars": [
    "x",
    "y",
    "a",
    "b",
    "c",
    "d",
    "e",
    "f",
    "r",
    "n",
    "m",
    "k",
    "i"
  ],
  "params": [
    "S",
    "q",
    "q_0",
    "q_1",
    "q_2",
    "q_i",
    "q_k",
    "g_0",
    "g_1",
    "g_q_k"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "varxvalue",
        "y": "varyvalue",
        "a": "firstnuma",
        "b": "secondnumb",
        "c": "thirdnumc",
        "d": "auxnumd",
        "e": "auxnume",
        "f": "auxnumf",
        "r": "remainder",
        "n": "powerindex",
        "m": "factorindex",
        "k": "binaryindex",
        "i": "genericidx",
        "S": "numberset",
        "q": "integerq",
        "q_0": "quotientzero",
        "q_1": "quotientone",
        "q_2": "quotienttwo",
        "q_i": "quotienti",
        "q_k": "quotientk",
        "g_0": "mapzero",
        "g_1": "mapone",
        "g_q_k": "mapqkval"
      },
      "question": "Let $numberset$ be a set of three, not necessarily distinct, positive integers.\nShow that one can transform $numberset$ into a set containing 0 by a finite\nnumber of applications of the following rule: Select two of the three\nintegers, say $varxvalue$ and $varyvalue$, where $varxvalue \\leq varyvalue$ and replace them with $2varxvalue$ and $varyvalue-varxvalue$.\n\n\\end{itemize}\n\\end{document}",
      "solution": "Solution 1 (attributed to Garth Payne). It suffices to show that \\( (firstnuma, secondnumb, thirdnumc) \\) with \\( 0<firstnuma \\leq secondnumb \\leq thirdnumc \\) can be transformed into \\( \\left(secondnumb^{\\prime}, firstnuma^{\\prime}, thirdnumc^{\\prime}\\right) \\), where \\( firstnuma^{\\prime} \\) is the remainder when \\( secondnumb \\) is divided by \\( firstnuma \\). (Hence any set with no zeros can be transformed into a set with a smaller element.)\n\nLet \\( secondnumb=integerq firstnuma+remainder \\) with \\( remainder<firstnuma \\); let \\( integerq=quotientzero+2 quotientone+4 quotienttwo+\\cdots+2^{binaryindex} quotientk \\) be the binary representation of \\( integerq \\) (so \\( quotienti=0 \\) or 1 , and \\( quotientk=1 \\) ). Define \\( mapzero(auxnumd, auxnume, auxnumf)=(2 auxnumd, auxnume, auxnumf-auxnumd) \\) and \\( mapone(auxnumd, auxnume, auxnumf)=(2 auxnumd, auxnume-auxnumd, auxnumf) \\). Then\n\\[\ng_{quotientk}\\left(\\cdots\\left(g_{quotientone}\\left(g_{quotientzero}(firstnuma, secondnumb, thirdnumc)\\right)\\right) \\cdots\\right)=\\left(secondnumb^{\\prime}, remainder, thirdnumc^{\\prime}\\right)\n\\]\ndescribes a sequence of legal moves.\n\nSolution 2. Let \\( firstnuma, secondnumb, thirdnumc \\) be the elements of \\( numberset \\). We prove the result by strong induction on \\( firstnuma+secondnumb+thirdnumc \\), a quantity that is preserved by applications of the rule. For the sake of obtaining a contradiction, assume that \\( numberset \\) cannot be transformed to a triple containing 0 .\n\nFirst we reduce to the case that exactly one of \\( firstnuma, secondnumb, thirdnumc \\) is odd. If two are odd, apply the rule with those two, and then none are odd. If none are odd, divide all numbers by 2 and apply the inductive hypothesis. If three are odd, apply the rule once, and exactly one is odd. Once exactly one is odd, this will remain so.\n\nWithout loss of generality, \\( firstnuma \\) is odd and \\( secondnumb \\) and \\( thirdnumc \\) are even. Let \\( 2^{powerindex} \\) be the highest power of 2 dividing \\( secondnumb+thirdnumc \\). We will describe a series of moves leaving us with \\( firstnuma^{\\prime}, secondnumb^{\\prime}, thirdnumc^{\\prime} \\), with \\( firstnuma^{\\prime} \\) odd and \\( 2^{powerindex+1} \\) dividing \\( secondnumb^{\\prime}+thirdnumc^{\\prime} \\). Repeating such a series of moves results in a triple \\( firstnuma^{\\prime \\prime}, secondnumb^{\\prime \\prime}, thirdnumc^{\\prime \\prime} \\) with \\( secondnumb^{\\prime \\prime}+thirdnumc^{\\prime \\prime} \\) divisible by \\( 2^{factorindex} \\) where \\( 2^{factorindex}>firstnuma+secondnumb+thirdnumc \\). Then since \\( secondnumb^{\\prime \\prime}, thirdnumc^{\\prime \\prime}>0 \\), we have \\( secondnumb^{\\prime \\prime}+thirdnumc^{\\prime \\prime} \\geq 2^{factorindex}>firstnuma+secondnumb+thirdnumc=firstnuma^{\\prime \\prime}+secondnumb^{\\prime \\prime}+thirdnumc^{\\prime \\prime} \\), contradicting \\( firstnuma^{\\prime \\prime}>0 \\).\n\nWe first apply a series of moves so that \\( secondnumb \\) and \\( thirdnumc \\) are divisible by different powers of 2 , and the one divisible by the smaller power of 2(secondnumb , say ) is also smaller. If \\( secondnumb \\) and \\( thirdnumc \\) have the same number of factors of 2 , then applying the rule to those two will yield both divisible by a higher power of 2 , or one will have fewer factors of 2 than the other. Since \\( secondnumb+thirdnumc \\) is constant here, after a finite number of applications of the rule, \\( secondnumb \\) and \\( thirdnumc \\) will not have the same number of factors of 2 . Also, possibly after some additional moves (on \\( secondnumb \\) and \\( thirdnumc \\) ), the one of \\( secondnumb \\) and \\( thirdnumc \\) divisible by the smaller power of 2 is also smaller.\n\nNow if \\( firstnuma>secondnumb \\), then apply the rule to \\( (firstnuma, secondnumb) \\); \\( firstnuma \\) remains odd, \\( secondnumb \\) is doubled, and \\( secondnumb+thirdnumc \\) is divisible by a higher power of 2 as desired.\n\nOn the other hand, if \\( firstnuma<secondnumb \\), apply the rule first to \\( (firstnuma, secondnumb) \\), and then to \\( (secondnumb-firstnuma, thirdnumc) \\). (Note that \\( thirdnumc>secondnumb>secondnumb-firstnuma \\).) The result is the triple \\( (2 firstnuma, 2 secondnumb-2 firstnuma, thirdnumc-secondnumb+firstnuma) \\). Now the odd number is \\( thirdnumc-secondnumb+firstnuma \\), and the sum of the even numbers is \\( 2 secondnumb \\), which has one more factor of 2 than \\( secondnumb+thirdnumc \\).\n\nSolution 3 (Dylan Thurston). As in Solution 2, we assume there is some triple \\( firstnuma, secondnumb, thirdnumc \\) that cannot be transformed to a triple containing 0 , and that exactly one of \\( firstnuma, secondnumb, thirdnumc \\) is odd. We give a recipe showing that if two of the triples are divisible by \\( 2^{powerindex} \\), then we can reach a state where two are divisible by \\( 2^{powerindex+1} \\); since the sum of the triple is constant, this eventually gives a contradiction.\n\nThe result is trivial if both of the multiples of \\( 2^{powerindex} \\) are already divisible by \\( 2^{powerindex+1} \\), or if neither are (just apply the rule once to that pair). So assume one is a multiple of \\( 2^{powerindex+1} \\), and the other is not, so the triple is (after renaming) \\( (firstnuma, secondnumb, thirdnumc) \\equiv\\left(0,2^{powerindex}, varxvalue\\right)\\left(\\bmod 2^{powerindex+1}\\right) \\) (where \\( varxvalue \\) is odd).\n\nWe can assume \\( firstnuma>thirdnumc \\). (Otherwise, apply the rule to ( \\( firstnuma, thirdnumc \\) ) repeatedly until this is so.) Then apply the rule to \\( (firstnuma, thirdnumc) \\), giving the triple \\( \\left(firstnuma^{\\prime}, secondnumb^{\\prime}, thirdnumc^{\\prime}\\right) \\equiv\\left(-varxvalue, 2^{powerindex}, 2 varxvalue\\right)\\left(\\bmod 2^{powerindex+1}\\right) \\).\n\nIf \\( firstnuma^{\\prime}<secondnumb^{\\prime} \\), then apply the rule to \\( \\left(firstnuma^{\\prime}, secondnumb^{\\prime}\\right) \\), giving \\( \\left(-2 varxvalue, 2^{powerindex}+varxvalue, 2 varxvalue\\right)\\left(\\bmod 2^{powerindex+1}\\right) \\). Repeated applications of the rule to \\( (2 varxvalue,-2 varxvalue) \\) eventually produce \\( (0,0)\\left(\\bmod 2^{powerindex+1}\\right) \\) (regardless of which is bigger at each stage).\n\nIf on the other hand \\( firstnuma^{\\prime}>secondnumb^{\\prime} \\), apply the rule to \\( \\left(firstnuma^{\\prime}, secondnumb^{\\prime}\\right) \\) giving \\( \\left(2^{powerindex}-varxvalue, 0,2 varxvalue\\right) \\) \\( \\left(\\bmod 2^{powerindex+1}\\right) \\). Apply the rule repeatedly to the 0 and \\( 2^{powerindex}-varxvalue \\) terms until the 0 is bigger, and then apply it once more to get \\( \\left(-2 varxvalue, 2^{powerindex}+varxvalue, 2 varxvalue\\right)\\left(\\bmod 2^{powerindex+1}\\right) \\). Again apply the rule repeatedly to \\( (2 varxvalue,-2 varxvalue) \\) to eventually produce \\( (0,0)\\left(\\bmod 2^{powerindex+1}\\right) \\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "mapleleaf",
        "y": "sandstone",
        "a": "pineapple",
        "b": "cardboard",
        "c": "firebrick",
        "d": "blueberry",
        "e": "chameleon",
        "f": "dragonfly",
        "r": "goldcrest",
        "n": "springbok",
        "m": "blackbird",
        "k": "lighthouse",
        "i": "kingfisher",
        "S": "semaphore",
        "q": "toadstool",
        "q_0": "carousel",
        "q_1": "rainstorm",
        "q_2": "gingerale",
        "q_i": "marshland",
        "q_k": "driftwood",
        "g_0": "buttercup",
        "g_1": "dandelion",
        "g_q_k": "cloudscape"
      },
      "question": "Let $semaphore$ be a set of three, not necessarily distinct, positive integers.\nShow that one can transform $semaphore$ into a set containing 0 by a finite\nnumber of applications of the following rule: Select two of the three\nintegers, say $mapleleaf$ and $sandstone$, where $mapleleaf \\leq sandstone$ and replace them with $2mapleleaf$ and $sandstone-mapleleaf$.",
      "solution": "Solution 1 (attributed to Garth Payne). It suffices to show that \\( (pineapple, cardboard, firebrick) \\) with \\( 0<pineapple \\leq cardboard \\leq firebrick \\) can be transformed into \\( \\left(cardboard^{\\prime}, pineapple^{\\prime}, firebrick^{\\prime}\\right) \\), where \\( pineapple^{\\prime} \\) is the remainder when \\( cardboard \\) is divided by \\( pineapple \\). (Hence any set with no zeros can be transformed into a set with a smaller element.)\n\nLet \\( cardboard=toadstool\\, pineapple+goldcrest \\) with \\( goldcrest<pineapple \\); let \\( toadstool=carousel+2 rainstorm+4 gingerale+\\cdots+2^{lighthouse} driftwood \\) be the binary representation of \\( toadstool \\) (so \\( marshland=0 \\) or 1 , and \\( driftwood=1 \\) ). Define \\( buttercup(blueberry, chameleon, dragonfly)=(2 blueberry, chameleon, dragonfly-blueberry) \\) and \\( dandelion(blueberry, chameleon, dragonfly)=(2 blueberry, chameleon-blueberry, dragonfly) \\). Then\n\\[\ncloudscape\\left(\\cdots\\left(g_{rainstorm}\\left(g_{carousel}(pineapple, cardboard, firebrick)\\right)\\right) \\cdots\\right)=\\left(cardboard^{\\prime}, goldcrest, firebrick^{\\prime}\\right)\n\\]\ndescribes a sequence of legal moves.\n\nSolution 2. Let \\( pineapple, cardboard, firebrick \\) be the elements of \\( semaphore \\). We prove the result by strong induction on \\( pineapple+cardboard+firebrick \\), a quantity that is preserved by applications of the rule. For the sake of obtaining a contradiction, assume that \\( semaphore \\) cannot be transformed to a triple containing 0.\n\nFirst we reduce to the case that exactly one of \\( pineapple, cardboard, firebrick \\) is odd. If two are odd, apply the rule with those two, and then none are odd. If none are odd, divide all numbers by 2 and apply the inductive hypothesis. If three are odd, apply the rule once, and exactly one is odd. Once exactly one is odd, this will remain so.\n\nWithout loss of generality, \\( pineapple \\) is odd and \\( cardboard \\) and \\( firebrick \\) are even. Let \\( 2^{springbok} \\) be the highest power of 2 dividing \\( cardboard+firebrick \\). We will describe a series of moves leaving us with \\( pineapple^{\\prime}, cardboard^{\\prime}, firebrick^{\\prime} \\), with \\( pineapple^{\\prime} \\) odd and \\( 2^{springbok+1} \\) dividing \\( cardboard^{\\prime}+firebrick^{\\prime} \\). Repeating such a series of moves results in a triple \\( pineapple^{\\prime \\prime}, cardboard^{\\prime \\prime}, firebrick^{\\prime \\prime} \\) with \\( cardboard^{\\prime \\prime}+firebrick^{\\prime \\prime} \\) divisible by \\( 2^{blackbird} \\) where \\( 2^{blackbird}>pineapple+cardboard+firebrick \\). Then since \\( cardboard^{\\prime \\prime}, firebrick^{\\prime \\prime}>0 \\), we have \\( cardboard^{\\prime \\prime}+firebrick^{\\prime \\prime} \\geq 2^{blackbird}>pineapple+cardboard+firebrick=pineapple^{\\prime \\prime}+cardboard^{\\prime \\prime}+firebrick^{\\prime \\prime} \\), contradicting \\( pineapple^{\\prime \\prime}>0 \\).\n\nWe first apply a series of moves so that \\( cardboard \\) and \\( firebrick \\) are divisible by different powers of 2, and the one divisible by the smaller power of 2 ( \\( cardboard \\), say ) is also smaller. If \\( cardboard \\) and \\( firebrick \\) have the same number of factors of 2, then applying the rule to those two will yield both divisible by a higher power of 2, or one will have fewer factors of 2 than the other. Since \\( cardboard+firebrick \\) is constant here, after a finite number of applications of the rule, \\( cardboard \\) and \\( firebrick \\) will not have the same number of factors of 2. Also, possibly after some additional moves (on \\( cardboard \\) and \\( firebrick \\) ), the one of \\( cardboard \\) and \\( firebrick \\) divisible by the smaller power of 2 is also smaller.\n\nNow if \\( pineapple>cardboard \\), then apply the rule to \\( (pineapple, cardboard) \\); \\( pineapple \\) remains odd, \\( cardboard \\) is doubled, and \\( cardboard+firebrick \\) is divisible by a higher power of 2 as desired.\n\nOn the other hand, if \\( pineapple<cardboard \\), apply the rule first to \\( (pineapple, cardboard) \\), and then to \\( (cardboard-pineapple, firebrick) \\). (Note that \\( firebrick>cardboard>cardboard-pineapple \\).) The result is the triple \\( (2 pineapple, 2 cardboard-2 pineapple, firebrick-cardboard+pineapple) \\). Now the odd number is \\( firebrick-cardboard+pineapple \\), and the sum of the even numbers is \\( 2 cardboard \\), which has one more factor of 2 than \\( cardboard+firebrick \\).\n\nSolution 3 (Dylan Thurston). As in Solution 2, we assume there is some triple \\( pineapple, cardboard, firebrick \\) that cannot be transformed to a triple containing 0, and that exactly one of \\( pineapple, cardboard, firebrick \\) is odd. We give a recipe showing that if two of the triples are divisible by \\( 2^{springbok} \\), then we can reach a state where two are divisible by \\( 2^{springbok+1} \\); since the sum of the triple is constant, this eventually gives a contradiction.\n\nThe result is trivial if both of the multiples of \\( 2^{springbok} \\) are already divisible by \\( 2^{springbok+1} \\), or if neither are (just apply the rule once to that pair). So assume one is a multiple of \\( 2^{springbok+1} \\), and the other is not, so the triple is (after renaming) \\( (pineapple, cardboard, firebrick) \\equiv\\left(0,2^{springbok}, mapleleaf\\right)\\left(\\bmod 2^{springbok+1}\\right) \\) (where \\( mapleleaf \\) is odd).\n\nWe can assume \\( pineapple>firebrick \\). (Otherwise, apply the rule to \\( (pineapple, firebrick) \\) repeatedly until this is so.) Then apply the rule to \\( (pineapple, firebrick) \\), giving the triple \\( \\left(pineapple^{\\prime}, cardboard^{\\prime}, firebrick^{\\prime}\\right) \\equiv\\left(-mapleleaf, 2^{springbok}, 2 mapleleaf\\right)\\left(\\bmod 2^{springbok+1}\\right) \\).\n\nIf \\( pineapple^{\\prime}<cardboard^{\\prime} \\), then apply the rule to \\( \\left(pineapple^{\\prime}, cardboard^{\\prime}\\right) \\), giving \\( \\left(-2 mapleleaf, 2^{springbok}+mapleleaf, 2 mapleleaf\\right)\\left(\\bmod 2^{springbok+1}\\right) \\). Repeated applications of the rule to \\( (2 mapleleaf,-2 mapleleaf) \\) eventually produce \\( (0,0)\\left(\\bmod 2^{springbok+1}\\right) \\) (regardless of which is bigger at each stage).\n\nIf on the other hand \\( pineapple^{\\prime}>cardboard^{\\prime} \\), apply the rule to \\( \\left(pineapple^{\\prime}, cardboard^{\\prime}\\right) \\) giving \\( \\left(2^{springbok}-mapleleaf, 0,2 mapleleaf\\right) \\) \\( \\left(\\bmod 2^{springbok+1}\\right) \\). Apply the rule repeatedly to the 0 and \\( 2^{springbok}-mapleleaf \\) terms until the 0 is bigger, and then apply it once more to get \\( \\left(-2 mapleleaf, 2^{springbok}+mapleleaf, 2 mapleleaf\\right)\\left(\\bmod 2^{springbok+1}\\right) \\). Again apply the rule repeatedly to \\( (2 mapleleaf,-2 mapleleaf) \\) to eventually produce \\( (0,0)\\left(\\bmod 2^{springbok+1}\\right) \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "greaterinteger",
        "y": "smallerinteger",
        "a": "nonelement",
        "b": "nonmember",
        "c": "outsider",
        "d": "knownvalue",
        "e": "fixedvalue",
        "f": "givennumber",
        "r": "divisorval",
        "n": "lowpower",
        "m": "smallindex",
        "k": "minindex",
        "i": "maxindex",
        "S": "infiniteset",
        "q": "remainderpart",
        "q_0": "zerobitnegator",
        "q_1": "onebitnegator",
        "q_2": "twobitnegator",
        "q_i": "genericbitnegator",
        "q_k": "kbitnegator",
        "g_0": "constantzero",
        "g_1": "constantone",
        "g_q_k": "constantqk"
      },
      "question": "Let $infiniteset$ be a set of three, not necessarily distinct, positive integers.\nShow that one can transform $infiniteset$ into a set containing 0 by a finite\nnumber of applications of the following rule: Select two of the three\nintegers, say $greaterinteger$ and $smallerinteger$, where $greaterinteger \\leq smallerinteger$ and replace them with $2greaterinteger$ and $smallerinteger-greaterinteger$.",
      "solution": "Solution 1 (attributed to Garth Payne). It suffices to show that \\( (nonelement, nonmember, outsider) \\) with \\( 0<nonelement \\leq nonmember \\leq outsider \\) can be transformed into \\( \\left(nonmember^{\\prime}, nonelement^{\\prime}, outsider^{\\prime}\\right) \\), where \\( nonelement^{\\prime} \\) is the remainder when \\( nonmember \\) is divided by \\( nonelement \\). (Hence any set with no zeros can be transformed into a set with a smaller element.)\n\nLet \\( nonmember=remainderpart\\,nonelement+divisorval \\) with \\( divisorval<nonelement \\); let \\( remainderpart=zerobitnegator+2\\,onebitnegator+4\\,twobitnegator+\\cdots+2^{minindex}\\,kbitnegator \\) be the binary representation of \\( remainderpart \\) (so \\( genericbitnegator=0 \\) or 1 , and \\( kbitnegator=1 \\) ). Define \\( constantzero(knownvalue, fixedvalue, givennumber)=(2\\,knownvalue, fixedvalue, givennumber-knownvalue) \\) and \\( constantone(knownvalue, fixedvalue, givennumber)=(2\\,knownvalue, fixedvalue-knownvalue, givennumber) \\). Then\n\\[\nconstantqk\\left(\\cdots\\left(g_{onebitnegator}\\left(g_{zerobitnegator}(nonelement, nonmember, outsider)\\right)\\right) \\cdots\\right)=\\left(nonmember^{\\prime}, divisorval, outsider^{\\prime}\\right)\n\\]\ndescribes a sequence of legal moves.\n\nSolution 2. Let \\( nonelement, nonmember, outsider \\) be the elements of \\( infiniteset \\). We prove the result by strong induction on \\( nonelement+nonmember+outsider \\), a quantity that is preserved by applications of the rule. For the sake of obtaining a contradiction, assume that \\( infiniteset \\) cannot be transformed to a triple containing 0 .\n\nFirst we reduce to the case that exactly one of \\( nonelement, nonmember, outsider \\) is odd. If two are odd, apply the rule with those two, and then none are odd. If none are odd, divide all numbers by 2 and apply the inductive hypothesis. If three are odd, apply the rule once, and exactly one is odd. Once exactly one is odd, this will remain so.\n\nWithout loss of generality, \\( nonelement \\) is odd and \\( nonmember \\) and \\( outsider \\) are even. Let \\( 2^{lowpower} \\) be the highest power of 2 dividing \\( nonmember+outsider \\). We will describe a series of moves leaving us with \\( nonelement^{\\prime}, nonmember^{\\prime}, outsider^{\\prime} \\), with \\( nonelement^{\\prime} \\) odd and \\( 2^{lowpower+1} \\) dividing \\( nonmember^{\\prime}+outsider^{\\prime} \\). Repeating such a series of moves results in a triple \\( nonelement^{\\prime \\prime}, nonmember^{\\prime \\prime}, outsider^{\\prime \\prime} \\) with \\( nonmember^{\\prime \\prime}+outsider^{\\prime \\prime} \\) divisible by \\( 2^{smallindex} \\) where \\( 2^{smallindex}>nonelement+nonmember+outsider \\). Then since \\( nonmember^{\\prime \\prime}, outsider^{\\prime \\prime}>0 \\), we have \\( nonmember^{\\prime \\prime}+outsider^{\\prime \\prime} \\geq 2^{smallindex}>nonelement+nonmember+outsider=nonelement^{\\prime \\prime}+nonmember^{\\prime \\prime}+outsider^{\\prime \\prime} \\), contradicting \\( nonelement^{\\prime \\prime}>0 \\).\n\nWe first apply a series of moves so that \\( nonmember \\) and \\( outsider \\) are divisible by different powers of 2 , and the one divisible by the smaller power of 2 (\\( nonmember \\), say ) is also smaller. If \\( nonmember \\) and \\( outsider \\) have the same number of factors of 2 , then applying the rule to those two will yield both divisible by a higher power of 2 , or one will have fewer factors of 2 than the other. Since \\( nonmember+outsider \\) is constant here, after a finite number of applications of the rule, \\( nonmember \\) and \\( outsider \\) will not have the same number of factors of 2 . Also, possibly after some additional moves (on \\( nonmember \\) and \\( outsider \\) ), the one of \\( nonmember \\) and \\( outsider \\) divisible by the smaller power of 2 is also smaller.\n\nNow if \\( nonelement>nonmember \\), then apply the rule to \\( (nonelement, nonmember) \\); \\( nonelement \\) remains odd, \\( nonmember \\) is doubled, and \\( nonmember+outsider \\) is divisible by a higher power of 2 as desired.\n\nOn the other hand, if \\( nonelement<nonmember \\), apply the rule first to \\( (nonelement, nonmember) \\), and then to \\( (nonmember-nonelement, outsider) \\). (Note that \\( outsider>nonmember>nonmember-nonelement \\).) The result is the triple \\( (2\\,nonelement, 2\\,nonmember-2\\,nonelement, outsider-nonmember+nonelement) \\). Now the odd number is \\( outsider-nonmember+nonelement \\), and the sum of the even numbers is \\( 2\\,nonmember \\), which has one more factor of 2 than \\( nonmember+outsider \\).\n\nSolution 3 (Dylan Thurston). As in Solution 2, we assume there is some triple \\( nonelement, nonmember, outsider \\) that cannot be transformed to a triple containing 0 , and that exactly one of \\( nonelement, nonmember, outsider \\) is odd. We give a recipe showing that if two of the triples are divisible by \\( 2^{lowpower} \\), then we can reach a state where two are divisible by \\( 2^{lowpower+1} \\); since the sum of the triple is constant, this eventually gives a contradiction.\n\nThe result is trivial if both of the multiples of \\( 2^{lowpower} \\) are already divisible by \\( 2^{lowpower+1} \\), or if neither are (just apply the rule once to that pair). So assume one is a multiple of \\( 2^{lowpower+1} \\), and the other is not, so the triple is (after renaming) \\( (nonelement, nonmember, outsider) \\equiv(0,2^{lowpower}, greaterinteger)\\pmod{2^{lowpower+1}} \\) (where \\( greaterinteger \\) is odd).\n\nWe can assume \\( nonelement>outsider \\). (Otherwise, apply the rule to (\\( nonelement, outsider \\)) repeatedly until this is so.) Then apply the rule to \\( (nonelement, outsider) \\), giving the triple \\( (nonelement^{\\prime}, nonmember^{\\prime}, outsider^{\\prime}) \\equiv(-greaterinteger, 2^{lowpower}, 2\\,greaterinteger)\\pmod{2^{lowpower+1}} \\).\n\nIf \\( nonelement^{\\prime}<nonmember^{\\prime} \\), then apply the rule to \\( (nonelement^{\\prime}, nonmember^{\\prime}) \\), giving \\( (-2\\,greaterinteger, 2^{lowpower}+greaterinteger, 2\\,greaterinteger)\\pmod{2^{lowpower+1}} \\). Repeated applications of the rule to \\( (2\\,greaterinteger,-2\\,greaterinteger) \\) eventually produce \\( (0,0)\\pmod{2^{lowpower+1}} \\) (regardless of which is bigger at each stage).\n\nIf on the other hand \\( nonelement^{\\prime}>nonmember^{\\prime} \\), apply the rule to \\( (nonelement^{\\prime}, nonmember^{\\prime}) \\) giving \\( (2^{lowpower}-greaterinteger, 0,2\\,greaterinteger) \\pmod{2^{lowpower+1}} \\). Apply the rule repeatedly to the 0 and \\( 2^{lowpower}-greaterinteger \\) terms until the 0 is bigger, and then apply it once more to get \\( (-2\\,greaterinteger, 2^{lowpower}+greaterinteger, 2\\,greaterinteger) \\pmod{2^{lowpower+1}} \\). Again apply the rule repeatedly to \\( (2\\,greaterinteger,-2\\,greaterinteger) \\) to eventually produce \\( (0,0)\\pmod{2^{lowpower+1}} \\)."
    },
    "garbled_string": {
      "map": {
        "x": "zmpqkhtg",
        "y": "rncvwoas",
        "a": "hdjslqwe",
        "b": "gkmtzopa",
        "c": "rusivnae",
        "d": "bcxqlyet",
        "e": "akjdoqwe",
        "f": "dmlskjha",
        "r": "sjdklqre",
        "n": "qpwornbz",
        "m": "ylgctdax",
        "k": "vihzjwpq",
        "i": "nskdjfqu",
        "S": "pqmrzvly",
        "q": "abefuskl",
        "q_0": "lzxqtwop",
        "q_1": "vpscgnzm",
        "q_2": "oubkrgat",
        "q_i": "pqwnaosl",
        "q_k": "xzmrclod",
        "g_0": "dfnqsvye",
        "g_1": "ibhjpyra",
        "g_q_k": "rmlogpax"
      },
      "question": "Let $pqmrzvly$ be a set of three, not necessarily distinct, positive integers.\nShow that one can transform $pqmrzvly$ into a set containing 0 by a finite\nnumber of applications of the following rule: Select two of the three\nintegers, say $zmpqkhtg$ and $rncvwoas$, where $zmpqkhtg \\leq rncvwoas$ and replace them with $2zmpqkhtg$ and $rncvwoas-zmpqkhtg$.",
      "solution": "Solution 1 (attributed to Garth Payne). It suffices to show that \\( (hdjslqwe, gkmtzopa, rusivnae) \\) with \\( 0<hdjslqwe \\leq gkmtzopa \\leq rusivnae \\) can be transformed into \\( \\left(gkmtzopa^{\\prime}, hdjslqwe^{\\prime}, rusivnae^{\\prime}\\right) \\), where \\( hdjslqwe^{\\prime} \\) is the remainder when \\( gkmtzopa \\) is divided by \\( hdjslqwe \\). (Hence any set with no zeros can be transformed into a set with a smaller element.)\n\nLet \\( gkmtzopa=abefuskl hdjslqwe+sjdklqre \\) with \\( sjdklqre<hdjslqwe \\); let \\( abefuskl=lzxqtwop+2 vpscgnzm+4 oubkrgat+\\cdots+2^{vihzjwpq} xzmrclod \\) be the binary representation of \\( abefuskl \\) (so \\( pqwnaosl=0 \\) or 1 , and \\( xzmrclod=1 \\) ). Define \\( dfnqsvye(bcxqlyet, akjdoqwe, dmlskjha)=(2 bcxqlyet, akjdoqwe, dmlskjha-bcxqlyet) \\) and \\( ibhjpyra(bcxqlyet, akjdoqwe, dmlskjha)=(2 bcxqlyet, akjdoqwe-bcxqlyet, dmlskjha) \\). Then\n\\[\nrmlogpax\\left(\\cdots\\left(g_{vpscgnzm}\\left(g_{lzxqtwop}(hdjslqwe, gkmtzopa, rusivnae)\\right)\\right)\\cdots\\right)=\\left(gkmtzopa^{\\prime}, sjdklqre, rusivnae^{\\prime}\\right)\n\\]\ndescribes a sequence of legal moves.\n\nSolution 2. Let \\( hdjslqwe, gkmtzopa, rusivnae \\) be the elements of \\( pqmrzvly \\). We prove the result by strong induction on \\( hdjslqwe+gkmtzopa+rusivnae \\), a quantity that is preserved by applications of the rule. For the sake of obtaining a contradiction, assume that \\( pqmrzvly \\) cannot be transformed to a triple containing 0.\n\nFirst we reduce to the case that exactly one of \\( hdjslqwe, gkmtzopa, rusivnae \\) is odd. If two are odd, apply the rule with those two, and then none are odd. If none are odd, divide all numbers by 2 and apply the inductive hypothesis. If three are odd, apply the rule once, and exactly one is odd. Once exactly one is odd, this will remain so.\n\nWithout loss of generality, \\( hdjslqwe \\) is odd and \\( gkmtzopa \\) and \\( rusivnae \\) are even. Let \\( 2^{qpwornbz} \\) be the highest power of 2 dividing \\( gkmtzopa+rusivnae \\). We will describe a series of moves leaving us with \\( hdjslqwe^{\\prime}, gkmtzopa^{\\prime}, rusivnae^{\\prime} \\), with \\( hdjslqwe^{\\prime} \\) odd and \\( 2^{qpwornbz+1} \\) dividing \\( gkmtzopa^{\\prime}+rusivnae^{\\prime} \\). Repeating such a series of moves results in a triple \\( hdjslqwe^{\\prime\\prime}, gkmtzopa^{\\prime\\prime}, rusivnae^{\\prime\\prime} \\) with \\( gkmtzopa^{\\prime\\prime}+rusivnae^{\\prime\\prime} \\) divisible by \\( 2^{ylgctdax} \\) where \\( 2^{ylgctdax}>hdjslqwe+gkmtzopa+rusivnae \\). Then since \\( gkmtzopa^{\\prime\\prime}, rusivnae^{\\prime\\prime}>0 \\), we have \\( gkmtzopa^{\\prime\\prime}+rusivnae^{\\prime\\prime} \\geq 2^{ylgctdax}>hdjslqwe+gkmtzopa+rusivnae=hdjslqwe^{\\prime\\prime}+gkmtzopa^{\\prime\\prime}+rusivnae^{\\prime\\prime} \\), contradicting \\( hdjslqwe^{\\prime\\prime}>0 \\).\n\nWe first apply a series of moves so that \\( gkmtzopa \\) and \\( rusivnae \\) are divisible by different powers of 2, and the one divisible by the smaller power of 2 \\( (gkmtzopa, \\text{ say }) \\) is also smaller. If \\( gkmtzopa \\) and \\( rusivnae \\) have the same number of factors of 2, then applying the rule to those two will yield both divisible by a higher power of 2, or one will have fewer factors of 2 than the other. Since \\( gkmtzopa+rusivnae \\) is constant here, after a finite number of applications of the rule, \\( gkmtzopa \\) and \\( rusivnae \\) will not have the same number of factors of 2. Also, possibly after some additional moves (on \\( gkmtzopa \\) and \\( rusivnae \\) ), the one of \\( gkmtzopa \\) and \\( rusivnae \\) divisible by the smaller power of 2 is also smaller.\n\nNow if \\( hdjslqwe>gkmtzopa \\), then apply the rule to \\( (hdjslqwe, gkmtzopa) \\); \\( hdjslqwe \\) remains odd, \\( gkmtzopa \\) is doubled, and \\( gkmtzopa+rusivnae \\) is divisible by a higher power of 2 as desired.\n\nOn the other hand, if \\( hdjslqwe<gkmtzopa \\), apply the rule first to \\( (hdjslqwe, gkmtzopa) \\), and then to \\( (gkmtzopa-hdjslqwe, rusivnae) \\). (Note that \\( rusivnae>gkmtzopa>gkmtzopa-hdjslqwe \\).) The result is the triple \\( (2 hdjslqwe, 2 gkmtzopa-2 hdjslqwe, rusivnae-gkmtzopa+hdjslqwe) \\). Now the odd number is \\( rusivnae-gkmtzopa+hdjslqwe \\), and the sum of the even numbers is \\( 2 gkmtzopa \\), which has one more factor of 2 than \\( gkmtzopa+rusivnae \\).\n\nSolution 3 (Dylan Thurston). As in Solution 2, we assume there is some triple \\( hdjslqwe, gkmtzopa, rusivnae \\) that cannot be transformed to a triple containing 0, and that exactly one of \\( hdjslqwe, gkmtzopa, rusivnae \\) is odd. We give a recipe showing that if two of the triples are divisible by \\( 2^{qpwornbz} \\), then we can reach a state where two are divisible by \\( 2^{qpwornbz+1} \\); since the sum of the triple is constant, this eventually gives a contradiction.\n\nThe result is trivial if both of the multiples of \\( 2^{qpwornbz} \\) are already divisible by \\( 2^{qpwornbz+1} \\), or if neither are (just apply the rule once to that pair). So assume one is a multiple of \\( 2^{qpwornbz+1} \\), and the other is not, so the triple is (after renaming) \\( (hdjslqwe, gkmtzopa, rusivnae) \\equiv(0,2^{qpwornbz}, zmpqkhtg)\\pmod{2^{qpwornbz+1}} \\) (where \\( zmpqkhtg \\) is odd).\n\nWe can assume \\( hdjslqwe>rusivnae \\). (Otherwise, apply the rule to \\( (hdjslqwe, rusivnae) \\) repeatedly until this is so.) Then apply the rule to \\( (hdjslqwe, rusivnae) \\), giving the triple \\( (hdjslqwe^{\\prime}, gkmtzopa^{\\prime}, rusivnae^{\\prime}) \\equiv(-zmpqkhtg, 2^{qpwornbz}, 2 zmpqkhtg)\\pmod{2^{qpwornbz+1}} \\).\n\nIf \\( hdjslqwe^{\\prime}<gkmtzopa^{\\prime} \\), then apply the rule to \\( (hdjslqwe^{\\prime}, gkmtzopa^{\\prime}) \\), giving \\( (-2 zmpqkhtg, 2^{qpwornbz}+zmpqkhtg, 2 zmpqkhtg)\\pmod{2^{qpwornbz+1}} \\). Repeated applications of the rule to \\( (2 zmpqkhtg,-2 zmpqkhtg) \\) eventually produce \\( (0,0)\\pmod{2^{qpwornbz+1}} \\) (regardless of which is bigger at each stage).\n\nIf on the other hand \\( hdjslqwe^{\\prime}>gkmtzopa^{\\prime} \\), apply the rule to \\( (hdjslqwe^{\\prime}, gkmtzopa^{\\prime}) \\) giving \\( (2^{qpwornbz}-zmpqkhtg, 0, 2 zmpqkhtg)\\pmod{2^{qpwornbz+1}} \\). Apply the rule repeatedly to the 0 and \\( 2^{qpwornbz}-zmpqkhtg \\) terms until the 0 is bigger, and then apply it once more to get \\( (-2 zmpqkhtg, 2^{qpwornbz}+zmpqkhtg, 2 zmpqkhtg)\\pmod{2^{qpwornbz+1}} \\). Again apply the rule repeatedly to \\( (2 zmpqkhtg,-2 zmpqkhtg) \\) to eventually produce \\( (0,0)\\pmod{2^{qpwornbz+1}} \\)."
    },
    "kernel_variant": {
      "question": "Let S be a multiset (repetitions allowed) that contains at least three positive integers.  A legal move consists of choosing two elements x and y of S with x \\le y and replacing them by the two integers 2x and y\\!-\n x (all other elements of S are left unchanged).\n\nProve that, after finitely many legal moves, the multiset S can always be transformed into one that contains the number 0.",
      "solution": "We first treat the case of a triple T={a,b,c} with 0<a\\le b\\le c and then show how the general statement follows.\n\n------------------------------------------------------------\n1.  A reduction procedure for a triple (a , b , c)\n------------------------------------------------------------\nProposition.  For every triple (a,b,c) of positive integers with a\\le b\\le c there exists a finite sequence of legal moves that sends the triple either directly to one containing 0 or to another triple whose smallest positive entry is strictly smaller than a.\n\nProof.\nWrite b=qa+r with 0\\le r<a and let q=q_{0}+2q_{1}+\\dots +2^{k}q_{k} be the binary expansion of q (q_{i}\\in\\{0,1\\}, q_{k}=1).\n\nFor a triple (d,e,f) define the two elementary legal moves\n\\[g_{0}(d,e,f):=(2d,\\,e,\\,f-d) \\qquad(\\text{pair }d\\text{ with }f),\\]\n\\[g_{1}(d,e,f):=(2d,\\,e-d,\\,f) \\qquad(\\text{pair }d\\text{ with }e).\\]\n\nStarting from (a_{0},b_{0},c_{0}):=(a,b,c) perform the (k+1) moves\n\\[(a_{i+1},b_{i+1},c_{i+1}) := g_{q_{i}}(a_{i},b_{i},c_{i})\\qquad(i=0,\\dots ,k).\\]\n(By construction the i-th move is g_{0} when q_{i}=0 and g_{1} when q_{i}=1.)\n\nWe now check three facts.\n\n(a)  Exact value of the first coordinate.\nBecause in every step the first entry is doubled,\n\\[\\boxed{\\;a_{i}=2^{i}a\\;}\\qquad(i=0,1,\\dots ,k+1).\\]\n\n(b)  Exact value of the second coordinate.\nPut Q_{i}:=q_{0}+2q_{1}+\\dots +2^{i-1}q_{i-1} (Q_{0}=0).  We claim\n\\[\\boxed{\\;b_{i}= b-Q_{i}a=(q-Q_{i})a+r\\;}\\qquad(i=0,1,\\dots ,k+1).\\]\nThe formula is true for i=0.  Assume it holds for i.\nIf q_{i}=0 the move g_{0} leaves the second coordinate unchanged, so the formula remains true for i+1 (and Q_{i+1}=Q_{i}).\nIf q_{i}=1, then g_{1} subtracts a_{i}=2^{i}a from b_{i}; on the other hand Q_{i+1}=Q_{i}+2^{i}.  Consequently\n\\[b_{i+1}=b_{i}-2^{i}a=b-(Q_{i}+2^{i})a=b-Q_{i+1}a,\\]\nproving the claim.\n\nSetting i=k+1 and using Q_{k+1}=q we obtain b_{k+1}=r.  Thus the last triple produced by the recipe is\n\\[(2^{k+1}a,\\;r,\\;c_{k+1}).\\]\nIf r=0, the triple already contains 0 and we are done.  Otherwise r<a, so the least positive element has strictly decreased.\n\n(c)  Legality of every prescribed move.\nA move g_{0} is legal iff the first entry that is used, a_{i}, does not exceed the third entry, c_{i}.  A move g_{1} is legal iff a_{i}\\le b_{i}.  We treat the two cases separately.\n\n•  The case q_{i}=1 (move g_{1}).  Because q_{i}=1 the binary expansion of q contains the term 2^{i}, hence q-Q_{i}\\ge 2^{i}.  By (b)\n\\[b_{i}=(q-Q_{i})a+r\\ge 2^{i}a=a_{i},\\]\nso a_{i}\\le b_{i} and g_{1} is legal.\n\n•  The case q_{i}=0 (move g_{0}).  Since q_{k}=1 there is an index j>i with q_{j}=1, whence \nq-Q_{i}\\ge 2^{i+1}.  Using the equality a+b+c=a_{i}+b_{i}+c_{i} and the expressions from (a) and (b) we get\n\\[c_{i}=c+(1-2^{i}+Q_{i})a\\;\\;\\text{and}\\;\\;c_{i}-a_{i}=c+(Q_{i}+1-2^{i+1})a.\\]\nBecause b\\le c one has c\\ge qa\\ge(2^{i+1}+Q_{i})a, so the right-hand side is non–negative and therefore a_{i}\\le c_{i}.  Hence g_{0} is legal.\n\nIn both cases the element y-x created by the move equals b_{i}\\! -\\! a_{i} or c_{i}\\! -\\! a_{i}; each difference is non–negative by the preceding inequalities, so the new entry is indeed positive.  This finishes the proof of the proposition.\n\\hfill□\n\n------------------------------------------------------------\n2.  From triples to an arbitrary multiset\n------------------------------------------------------------\nLet S be a multiset of n\\ge3 positive integers and list its elements in non-decreasing order x_{1}\\le x_{2}\\le\\dots \\le x_{n}.  \n\n•  If x_{1}=x_{2}, a single legal move on the pair (x_{1},x_{2}) immediately creates a 0 and we are done.\n\n•  Otherwise x_{1}<x_{2}.  Choose any third element of S (say x_{n}) and apply the construction of Section 1 to the triple (x_{1},x_{2},x_{n}).  According to the proposition, after finitely many moves we arrive either at a multiset already containing 0 or at one in which the minimal positive entry is some r with 0<r<x_{1}.\n\nDuring those moves every element of the multiset stays positive, hence the minimum of S strictly decreases unless 0 appears.  The minimum is a non-negative integer, so it can drop only finitely many times.  Consequently the procedure must eventually reach the situation where the minimum equals 0, i.e. S contains the desired element 0.\n\\hfill□\n\n------------------------------------------------------------\n3.  Additional remarks\n------------------------------------------------------------\n(1)  The method is completely explicit: at each stage take the two smallest numbers together with any third one, write their ratio in binary and perform the corresponding sequence of moves g_{0}/g_{1}.\n\n(2)  The proof uses only the fact that the minimum of the multiset decreases whenever 0 has not yet appeared; no global invariant other than positivity is required.",
      "_meta": {
        "core_steps": [
          "View the allowed move (x,y)→(2x, y−x) as a way to replace (a,b) by (b−ka, a) for suitable k=2^i (Euclidean-algorithm step).",
          "Write the quotient q=⌊b/a⌋ in binary and choose a sequence of moves g0 / g1 that successively subtract 2^i a from b, yielding (r, b′, c′) with r=b mod a.",
          "Because r<a, the minimum entry strictly decreases; repeating the process emulates the Euclidean algorithm on successive pairs.",
          "The decreasing-minimum process terminates when the remainder is 0, producing a triple that contains 0."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Cardinality of the multiset (presently fixed at three numbers); any size ≥3 would let the same pairwise Euclidean-style reduction work by repeatedly focusing on one chosen pair.",
            "original": "three"
          },
          "slot2": {
            "description": "The requirement ‘select x and y with x≤y’; the inequality can be dropped (or replaced by x≥y) since we may always relabel the chosen pair before applying the move.",
            "original": "x ≤ y"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}