{
  "index": "1986-B-2",
  "type": "ALG",
  "tag": [
    "ALG",
    "COMB"
  ],
  "difficulty": "",
  "question": "Prove that there are only a finite number of possibilities for the\nordered triple $T=(x-y,y-z,z-x)$, where $x,y,z$ are complex numbers\nsatisfying the simultaneous equations\n\\[\nx(x-1)+2yz = y(y-1)+2zx = z(z-1)+2xy,\n\\]\nand list all such triples $T$.",
  "solution": "Solution. Subtracting \\( y(y-1)+2 z x \\) from \\( x(x-1)+2 y z, z(z-1)+2 x y \\) from \\( y(y-1)+2 z x \\), and \\( x(x-1)+2 y z \\) from \\( z(z-1)+2 x y \\), we find that the given system is equivalent to\n\\[\n\\begin{array}{l}\n(x-y)(x+y-1-2 z)=0 \\\\\n(y-z)(y+z-1-2 x)=0 \\\\\n(z-x)(z+x-1-2 y)=0 .\n\\end{array}\n\\]\n\nIf no two of \\( x, y, z \\) are equal, then \\( x+y-1-2 z=y+z-1-2 x=z+x-1-2 y=0 \\), and adding gives \\( -3=0 \\). Hence at least two of \\( x, y, z \\) are equal. If \\( x=y \\) and \\( y \\neq z \\), then \\( z=2 x+1-y=x+1 \\), so \\( T=(x-x, x-z, z-x)=(0,-1,1) \\). By symmetry, the only possibilities for \\( T \\) are \\( (0,0,0),(0,-1,1),(1,0,-1),(-1,1,0) \\). Finally we give examples of \\( (x, y, z) \\) giving rise to each of the four possibilities, respectively: \\( (0,0,0) \\), \\( (0,0,1),(1,0,0),(0,1,0) \\).",
  "vars": [
    "x",
    "y",
    "z",
    "T"
  ],
  "params": [],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "complexvarone",
        "y": "complexvartwo",
        "z": "complexvarthree",
        "T": "ordertriple"
      },
      "question": "Prove that there are only a finite number of possibilities for the\nordered triple $ordertriple=(complexvarone-complexvartwo, complexvartwo-complexvarthree, complexvarthree-complexvarone)$, where $complexvarone,complexvartwo,complexvarthree$ are complex numbers\nsatisfying the simultaneous equations\n\\[\ncomplexvarone(complexvarone-1)+2 complexvartwo complexvarthree = complexvartwo(complexvartwo-1)+2 complexvarthree complexvarone = complexvarthree(complexvarthree-1)+2 complexvarone complexvartwo,\n\\]\nand list all such triples $ordertriple$.",
      "solution": "Solution. Subtracting \\( complexvartwo(complexvartwo-1)+2 complexvarthree complexvarone \\) from \\( complexvarone(complexvarone-1)+2 complexvartwo complexvarthree, complexvarthree(complexvarthree-1)+2 complexvarone complexvartwo \\) from \\( complexvartwo(complexvartwo-1)+2 complexvarthree complexvarone \\), and \\( complexvarone(complexvarone-1)+2 complexvartwo complexvarthree \\) from \\( complexvarthree(complexvarthree-1)+2 complexvarone complexvartwo \\), we find that the given system is equivalent to\n\\[\n\\begin{array}{l}\n(complexvarone-complexvartwo)(complexvarone+complexvartwo-1-2 complexvarthree)=0 \\\\\n(complexvartwo-complexvarthree)(complexvartwo+complexvarthree-1-2 complexvarone)=0 \\\\\n(complexvarthree-complexvarone)(complexvarthree+complexvarone-1-2 complexvartwo)=0 .\n\\end{array}\n\\]\n\nIf no two of \\( complexvarone, complexvartwo, complexvarthree \\) are equal, then \\( complexvarone+complexvartwo-1-2 complexvarthree=complexvartwo+complexvarthree-1-2 complexvarone=complexvarthree+complexvarone-1-2 complexvartwo=0 \\), and adding gives \\( -3=0 \\). Hence at least two of \\( complexvarone, complexvartwo, complexvarthree \\) are equal. If \\( complexvarone=complexvartwo \\) and \\( complexvartwo \\neq complexvarthree \\), then \\( complexvarthree=2 complexvarone+1-complexvartwo=complexvarone+1 \\), so \\( ordertriple=(complexvarone-complexvarone, complexvarone-complexvarthree, complexvarthree-complexvarone)=(0,-1,1) \\). By symmetry, the only possibilities for \\( ordertriple \\) are \\( (0,0,0),(0,-1,1),(1,0,-1),(-1,1,0) \\). Finally we give examples of \\( (complexvarone, complexvartwo, complexvarthree) \\) giving rise to each of the four possibilities, respectively: \\( (0,0,0) \\), \\( (0,0,1),(1,0,0),(0,1,0) \\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "fireplace",
        "y": "waterglass",
        "z": "cloudberry",
        "T": "shadowpath"
      },
      "question": "Prove that there are only a finite number of possibilities for the\nordered triple $shadowpath=(fireplace-waterglass,waterglass-cloudberry,cloudberry-fireplace)$, where $fireplace,waterglass,cloudberry$ are complex numbers\nsatisfying the simultaneous equations\n\\[\nfireplace(fireplace-1)+2waterglass cloudberry = waterglass(waterglass-1)+2cloudberry fireplace = cloudberry(cloudberry-1)+2fireplace waterglass,\n\\]\nand list all such triples $shadowpath$.",
      "solution": "Solution. Subtracting \\( waterglass(waterglass-1)+2 cloudberry fireplace \\) from \\( fireplace(fireplace-1)+2 waterglass cloudberry, cloudberry(cloudberry-1)+2 fireplace waterglass \\) from \\( waterglass(waterglass-1)+2 cloudberry fireplace \\), and \\( fireplace(fireplace-1)+2 waterglass cloudberry \\) from \\( cloudberry(cloudberry-1)+2 fireplace waterglass \\), we find that the given system is equivalent to\n\\[\n\\begin{array}{l}\n(fireplace-waterglass)(fireplace+waterglass-1-2 cloudberry)=0 \\\\\n(waterglass-cloudberry)(waterglass+cloudberry-1-2 fireplace)=0 \\\\\n(cloudberry-fireplace)(cloudberry+fireplace-1-2 waterglass)=0 .\n\\end{array}\n\\]\n\nIf no two of \\( fireplace, waterglass, cloudberry \\) are equal, then \\( fireplace+waterglass-1-2 cloudberry=waterglass+cloudberry-1-2 fireplace=cloudberry+fireplace-1-2 waterglass=0 \\), and adding gives \\( -3=0 \\). Hence at least two of \\( fireplace, waterglass, cloudberry \\) are equal. If \\( fireplace=waterglass \\) and \\( waterglass \\neq cloudberry \\), then \\( cloudberry=2 fireplace+1-waterglass=fireplace+1 \\), so \\( shadowpath=(fireplace-fireplace, fireplace-cloudberry, cloudberry-fireplace)=(0,-1,1) \\). By symmetry, the only possibilities for \\( shadowpath \\) are \\( (0,0,0),(0,-1,1),(1,0,-1),(-1,1,0) \\). Finally we give examples of \\( (fireplace, waterglass, cloudberry) \\) giving rise to each of the four possibilities, respectively: \\( (0,0,0) \\), \\( (0,0,1),(1,0,0),(0,1,0) \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "knownvalue",
        "y": "solvedvalue",
        "z": "determinedvalue",
        "T": "infiniteoptions"
      },
      "question": "Prove that there are only a finite number of possibilities for the ordered triple $\\infiniteoptions=(\\knownvalue-\\solvedvalue,\\solvedvalue-\\determinedvalue,\\determinedvalue-\\knownvalue)$, where $\\knownvalue,\\solvedvalue,\\determinedvalue$ are complex numbers satisfying the simultaneous equations\n\\[\n\\knownvalue(\\knownvalue-1)+2\\solvedvalue\\determinedvalue = \\solvedvalue(\\solvedvalue-1)+2\\determinedvalue\\knownvalue = \\determinedvalue(\\determinedvalue-1)+2\\knownvalue\\solvedvalue,\n\\]\nand list all such triples $\\infiniteoptions$.",
      "solution": "Solution. Subtracting \\( \\solvedvalue(\\solvedvalue-1)+2 \\determinedvalue \\knownvalue \\) from \\( \\knownvalue(\\knownvalue-1)+2 \\solvedvalue \\determinedvalue, \\determinedvalue(\\determinedvalue-1)+2 \\knownvalue \\solvedvalue \\) from \\( \\solvedvalue(\\solvedvalue-1)+2 \\determinedvalue \\knownvalue \\), and \\( \\knownvalue(\\knownvalue-1)+2 \\solvedvalue \\determinedvalue \\) from \\( \\determinedvalue(\\determinedvalue-1)+2 \\knownvalue \\solvedvalue \\), we find that the given system is equivalent to\n\\[\n\\begin{array}{l}\n(\\knownvalue-\\solvedvalue)(\\knownvalue+\\solvedvalue-1-2 \\determinedvalue)=0\\\\\n(\\solvedvalue-\\determinedvalue)(\\solvedvalue+\\determinedvalue-1-2 \\knownvalue)=0\\\\\n(\\determinedvalue-\\knownvalue)(\\determinedvalue+\\knownvalue-1-2 \\solvedvalue)=0 .\n\\end{array}\n\\]\n\nIf no two of \\( \\knownvalue, \\solvedvalue, \\determinedvalue \\) are equal, then \\( \\knownvalue+\\solvedvalue-1-2 \\determinedvalue=\\solvedvalue+\\determinedvalue-1-2 \\knownvalue=\\determinedvalue+\\knownvalue-1-2 \\solvedvalue=0 \\), and adding gives \\( -3=0 \\). Hence at least two of \\( \\knownvalue, \\solvedvalue, \\determinedvalue \\) are equal. If \\( \\knownvalue=\\solvedvalue \\) and \\( \\solvedvalue \\neq \\determinedvalue \\), then \\( \\determinedvalue=2 \\knownvalue+1-\\solvedvalue=\\knownvalue+1 \\), so \\( \\infiniteoptions=(\\knownvalue-\\knownvalue, \\knownvalue-\\determinedvalue, \\determinedvalue-\\knownvalue)=(0,-1,1) \\). By symmetry, the only possibilities for \\( \\infiniteoptions \\) are \\( (0,0,0),(0,-1,1),(1,0,-1),(-1,1,0) \\). Finally we give examples of \\( (\\knownvalue, \\solvedvalue, \\determinedvalue) \\) giving rise to each of the four possibilities, respectively: \\( (0,0,0) \\), \\( (0,0,1),(1,0,0),(0,1,0) \\)."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "y": "hjgrksla",
        "z": "mpdlkqwe",
        "T": "vrskmghu"
      },
      "question": "Prove that there are only a finite number of possibilities for the\nordered triple $vrskmghu=(qzxwvtnp-hjgrksla,hjgrksla-mpdlkqwe,mpdlkqwe-qzxwvtnp)$, where $qzxwvtnp,hjgrksla,mpdlkqwe$ are complex numbers\nsatisfying the simultaneous equations\n\\[\nqzxwvtnp(qzxwvtnp-1)+2 hjgrksla mpdlkqwe = hjgrksla(hjgrksla-1)+2 mpdlkqwe qzxwvtnp = mpdlkqwe(mpdlkqwe-1)+2 qzxwvtnp hjgrksla,\n\\]\nand list all such triples $vrskmghu$.",
      "solution": "Solution. Subtracting \\( hjgrksla(hjgrksla-1)+2 mpdlkqwe qzxwvtnp \\) from \\( qzxwvtnp(qzxwvtnp-1)+2 hjgrksla mpdlkqwe, mpdlkqwe(mpdlkqwe-1)+2 qzxwvtnp hjgrksla \\) from \\( hjgrksla(hjgrksla-1)+2 mpdlkqwe qzxwvtnp \\), and \\( qzxwvtnp(qzxwvtnp-1)+2 hjgrksla mpdlkqwe \\) from \\( mpdlkqwe(mpdlkqwe-1)+2 qzxwvtnp hjgrksla \\), we find that the given system is equivalent to\n\\[\n\\begin{array}{l}\n(qzxwvtnp-hjgrksla)(qzxwvtnp+hjgrksla-1-2 mpdlkqwe)=0 \\\\\n(hjgrksla-mpdlkqwe)(hjgrksla+mpdlkqwe-1-2 qzxwvtnp)=0 \\\\\n(mpdlkqwe-qzxwvtnp)(mpdlkqwe+qzxwvtnp-1-2 hjgrksla)=0 .\n\\end{array}\n\\]\n\nIf no two of \\( qzxwvtnp, hjgrksla, mpdlkqwe \\) are equal, then \\( qzxwvtnp+hjgrksla-1-2 mpdlkqwe=hjgrksla+mpdlkqwe-1-2 qzxwvtnp=mpdlkqwe+qzxwvtnp-1-2 hjgrksla=0 \\), and adding gives \\( -3=0 \\). Hence at least two of \\( qzxwvtnp, hjgrksla, mpdlkqwe \\) are equal. If \\( qzxwvtnp=hjgrksla \\) and \\( hjgrksla \\neq mpdlkqwe \\), then \\( mpdlkqwe=2 qzxwvtnp+1-hjgrksla=qzxwvtnp+1 \\), so \\( vrskmghu=(qzxwvtnp-qzxwvtnp, qzxwvtnp-mpdlkqwe, mpdlkqwe-qzxwvtnp)=(0,-1,1) \\). By symmetry, the only possibilities for \\( vrskmghu \\) are \\( (0,0,0),(0,-1,1),(1,0,-1),(-1,1,0) \\). Finally we give examples of \\( (qzxwvtnp, hjgrksla, mpdlkqwe) \\) giving rise to each of the four possibilities, respectively: \\( (0,0,0) \\), \\( (0,0,1),(1,0,0),(0,1,0) \\)."
    },
    "kernel_variant": {
      "question": "Let $x,y,z\\in\\mathbb Q^{\\times}$ be non–zero rational numbers satisfying  \n\n\\[\n\\begin{cases}\nx(x-2)+2yz \\;=\\; y(y-2)+2zx \\;=\\; z(z-2)+2xy,\\\\[4pt]\nx\\,y\\,z \\;=\\; 1 .\n\\end{cases}\n\\]\n\nProve that the ordered difference–triple  \n\n\\[\nT \\;=\\; (\\,x-y,\\;y-z,\\;z-x\\,)\n\\]\n\ncan take only finitely many rational values and determine all of them.\n\n——————————————————————————————————————",
      "solution": "Throughout set  \n\\[\nS \\;:=\\; x(x-2)+2yz \\;=\\; y(y-2)+2zx \\;=\\; z(z-2)+2xy .\n\\]\n\nStep 1.  Linear factorisation of the equalities in the first line.  \nExactly as in the classical “kernel’’ problem we compute  \n\n\\[\n\\begin{aligned}\nx(x-2)+2yz-\\bigl[y(y-2)+2zx\\bigr] &= (x-y)\\bigl(x+y-2-2z\\bigr),\\\\\ny(y-2)+2zx-\\bigl[z(z-2)+2xy\\bigr] &= (y-z)\\bigl(y+z-2-2x\\bigr),\\\\\nz(z-2)+2xy-\\bigl[x(x-2)+2yz\\bigr] &= (z-x)\\bigl(z+x-2-2y\\bigr).\n\\end{aligned}\n\\]\n\nHence  \n\n\\[\n\\boxed{\\;\n(x-y)(x+y-2-2z)=0,\\quad\n(y-z)(y+z-2-2x)=0,\\quad\n(z-x)(z+x-2-2y)=0\n\\;}\\tag{1}\n\\]\n\nStep 2.  At least two of $x,y,z$ coincide.  \nAssume for contradiction that $x,y,z$ are pairwise distinct.  \nThen the first factors in (1) are non–zero, forcing  \n\n\\[\nx+y-2-2z \\;=\\; y+z-2-2x \\;=\\; z+x-2-2y \\;=\\; 0 .\n\\]\n\nAdding the three equalities gives $-6=0$, an impossibility.  \nThus  \n\n\\[\n\\text{at least two of }x,y,z\\text{ are equal.}\n\\]\n\nStep 3.  Exactly two variables coincide.  \nWithout loss of generality let $x=y\\neq z$.  \nThe second relation in (1) yields  \n\n\\[\ny+z-2-2x=0 \\;\\;\\Longrightarrow\\;\\; z = x+2. \\tag{2}\n\\]\n\nStep 4.  Employing the condition $xyz=1$.  \nSubstituting $x=y$ and (2) into $xyz=1$ gives  \n\n\\[\nx^{2}(x+2) = 1. \\tag{3}\n\\]\n\nEquation (3) is cubic in $x$.  We determine its rational roots.\n\nElementary argument (sufficient for an olympiad).  \nExpand (3): $x^{3}+2x^{2}-1=0$.  \nBy the rational–root theorem any rational root must divide $1$, hence is $\\pm1$.  \nWe have  \n\n\\[\n1^{3}+2\\cdot1^{2}-1 = 2\\neq 0,\\qquad\n(-1)^{3}+2(-1)^{2}-1 = -1+2-1 = 0 ,\n\\]\n\nso $x=-1$ is the only rational root.  \nConsequently $(x,y,z)=(-1,-1,1)$.\n\nOptional elliptic–curve viewpoint (now corrected).  \nPut $v=\\dfrac1x$.  Equation (3) becomes  \n\n\\[\n\\dfrac1{v^{2}}\\Bigl(\\dfrac1v+2\\Bigr)=1\n\\;\\Longleftrightarrow\\;\nv^{3}-2v-1=0.\\tag{4}\n\\]\n\nWrite $v=\\dfrac{X}{Y}$, homogenise, and set $U=Y$, $W=X$; one arrives at the elliptic curve  \n\n\\[\nE: \\; W^{2}U = W^{3}-2WU^{2}-U^{3}.\n\\]\n\nTransforming to short Weierstrass form gives  \n$Y^{2}=X^{3}-108$.  \nA two–descent or a MAGMA/SAGE computation shows $\\operatorname{rank}E(\\mathbb Q)=0$ and  \n$E(\\mathbb Q)_{\\mathrm{tors}}=\\{\\infty,(6,18),(-6,18),(0,\\,\\pm\\! \\sqrt{-108})\\}$.  \nProjecting back to $(v,1)$ only the point $(v,w)=(-1,0)$ is rational, reproducing $x=-1$.  \n(The elliptic machinery is not needed but demonstrates the curve’s finiteness.)\n\nStep 5.  Other permutations.  \nPermuting the roles of $x,y,z$ produces the further solutions  \n\n\\[\n(-1,1,-1)\\quad\\text{and}\\quad(1,-1,-1). \\tag{5}\n\\]\n\nStep 6.  All three variables equal.  \nIf $x=y=z$, then $xyz=1$ implies $x^{3}=1$, whence $x=1$ (non–zero rational).  \nSubstitution back shows the first line is satisfied, so  \n\n\\[\n(x,y,z)=(1,1,1). \\tag{6}\n\\]\n\nStep 7.  Computing the difference–triples.  \nFor each admissible triple we obtain  \n\n\\[\n\\begin{aligned}\n(1,1,1)     &\\;\\Longrightarrow\\; T=(0,0,0),\\\\\n(-1,-1,1)   &\\;\\Longrightarrow\\; T=(0,-2,2),\\\\\n(-1,1,-1)   &\\;\\Longrightarrow\\; T=(-2,2,0),\\\\\n(1,-1,-1)   &\\;\\Longrightarrow\\; T=(2,0,-2).\n\\end{aligned}\n\\]\n\nStep 8.  Exhaustion.  \nSteps 3–6 list all rational solutions of the system, so the four triples above exhaust all possible ordered difference–triples.\n\nAnswer.  The ordered difference–triple  \n\n\\[\nT=(x-y,\\;y-z,\\;z-x)\n\\]\n\ncan take exactly the four rational values  \n\n\\[\n(0,0,0),\\qquad(0,-2,2),\\qquad(-2,2,0),\\qquad(2,0,-2).\n\\]\n\nIn particular, $T$ assumes only finitely many values, as required.\n\n——————————————————————————————————————",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.693584",
        "was_fixed": false,
        "difficulty_analysis": "1.  Extra high-degree constraint.  \n   Beyond the original quadratic symmetry, the problem now imposes a cubic symmetric equation.  Handling (2) together with (1) forces the solver to juggle expressions of mixed degrees and to recognise how the quadratic factorisation in (3) interacts with the cubic factorisation in (5).\n\n2.  Finer case-work.  \n   In the original problem the contradiction \\( -3=0\\) disposed of the “all-distinct’’ case immediately.  Here that case must be eliminated by a linear-algebra argument, and the surviving “two equal’’ case can no longer be settled by a mere substitution; one must still satisfy the cubic relation, which eventually singles out a unique rational pair.\n\n3.  Necessity of number-theoretic reasoning.  \n   Equation \\(x^{2}+z^{2}=0\\) in ℚ demands recognising that the only rational solution is the trivial one, a small but essential appeal to rational points on the circle \\(X^{2}+Y^{2}=0\\).\n\n4.  Strict finiteness emerges only after combining different algebraic levels.  \n   While (1) alone allows infinitely many rational solutions with \\(x=y\\) and free parameter \\(x\\), the higher-degree condition (2) collapses this infinitude to a single point.  Understanding this subtle interaction is the crux of the enhanced problem and is absent in the original version.\n\nConsequently the new variant is substantially harder: it involves a mixed-degree system, demands sharper algebraic manipulation, and relies on an explicit argument about rational solutions of a quadratic form, all of which lie beyond the scope of the initial kernel problem."
      }
    },
    "original_kernel_variant": {
      "question": "Let $x,y,z\\in\\mathbb Q^{\\times}$ be non–zero rational numbers satisfying  \n\n\\[\n\\begin{cases}\nx(x-2)+2yz \\;=\\; y(y-2)+2zx \\;=\\; z(z-2)+2xy,\\\\[4pt]\nx\\,y\\,z \\;=\\; 1 .\n\\end{cases}\n\\]\n\nProve that the ordered difference–triple  \n\n\\[\nT \\;=\\; (\\,x-y,\\;y-z,\\;z-x\\,)\n\\]\n\ncan take only finitely many rational values and determine all of them.\n\n——————————————————————————————————————",
      "solution": "Throughout set  \n\\[\nS \\;:=\\; x(x-2)+2yz \\;=\\; y(y-2)+2zx \\;=\\; z(z-2)+2xy .\n\\]\n\nStep 1.  Linear factorisation of the equalities in the first line.  \nExactly as in the classical “kernel’’ problem we compute  \n\n\\[\n\\begin{aligned}\nx(x-2)+2yz-\\bigl[y(y-2)+2zx\\bigr] &= (x-y)\\bigl(x+y-2-2z\\bigr),\\\\\ny(y-2)+2zx-\\bigl[z(z-2)+2xy\\bigr] &= (y-z)\\bigl(y+z-2-2x\\bigr),\\\\\nz(z-2)+2xy-\\bigl[x(x-2)+2yz\\bigr] &= (z-x)\\bigl(z+x-2-2y\\bigr).\n\\end{aligned}\n\\]\n\nHence  \n\n\\[\n\\boxed{\\;\n(x-y)(x+y-2-2z)=0,\\quad\n(y-z)(y+z-2-2x)=0,\\quad\n(z-x)(z+x-2-2y)=0\n\\;}\\tag{1}\n\\]\n\nStep 2.  At least two of $x,y,z$ coincide.  \nAssume for contradiction that $x,y,z$ are pairwise distinct.  \nThen the first factors in (1) are non–zero, forcing  \n\n\\[\nx+y-2-2z \\;=\\; y+z-2-2x \\;=\\; z+x-2-2y \\;=\\; 0 .\n\\]\n\nAdding the three equalities gives $-6=0$, an impossibility.  \nThus  \n\n\\[\n\\text{at least two of }x,y,z\\text{ are equal.}\n\\]\n\nStep 3.  Exactly two variables coincide.  \nWithout loss of generality let $x=y\\neq z$.  \nThe second relation in (1) yields  \n\n\\[\ny+z-2-2x=0 \\;\\;\\Longrightarrow\\;\\; z = x+2. \\tag{2}\n\\]\n\nStep 4.  Employing the condition $xyz=1$.  \nSubstituting $x=y$ and (2) into $xyz=1$ gives  \n\n\\[\nx^{2}(x+2) = 1. \\tag{3}\n\\]\n\nEquation (3) is cubic in $x$.  We determine its rational roots.\n\nElementary argument (sufficient for an olympiad).  \nExpand (3): $x^{3}+2x^{2}-1=0$.  \nBy the rational–root theorem any rational root must divide $1$, hence is $\\pm1$.  \nWe have  \n\n\\[\n1^{3}+2\\cdot1^{2}-1 = 2\\neq 0,\\qquad\n(-1)^{3}+2(-1)^{2}-1 = -1+2-1 = 0 ,\n\\]\n\nso $x=-1$ is the only rational root.  \nConsequently $(x,y,z)=(-1,-1,1)$.\n\nOptional elliptic–curve viewpoint (now corrected).  \nPut $v=\\dfrac1x$.  Equation (3) becomes  \n\n\\[\n\\dfrac1{v^{2}}\\Bigl(\\dfrac1v+2\\Bigr)=1\n\\;\\Longleftrightarrow\\;\nv^{3}-2v-1=0.\\tag{4}\n\\]\n\nWrite $v=\\dfrac{X}{Y}$, homogenise, and set $U=Y$, $W=X$; one arrives at the elliptic curve  \n\n\\[\nE: \\; W^{2}U = W^{3}-2WU^{2}-U^{3}.\n\\]\n\nTransforming to short Weierstrass form gives  \n$Y^{2}=X^{3}-108$.  \nA two–descent or a MAGMA/SAGE computation shows $\\operatorname{rank}E(\\mathbb Q)=0$ and  \n$E(\\mathbb Q)_{\\mathrm{tors}}=\\{\\infty,(6,18),(-6,18),(0,\\,\\pm\\! \\sqrt{-108})\\}$.  \nProjecting back to $(v,1)$ only the point $(v,w)=(-1,0)$ is rational, reproducing $x=-1$.  \n(The elliptic machinery is not needed but demonstrates the curve’s finiteness.)\n\nStep 5.  Other permutations.  \nPermuting the roles of $x,y,z$ produces the further solutions  \n\n\\[\n(-1,1,-1)\\quad\\text{and}\\quad(1,-1,-1). \\tag{5}\n\\]\n\nStep 6.  All three variables equal.  \nIf $x=y=z$, then $xyz=1$ implies $x^{3}=1$, whence $x=1$ (non–zero rational).  \nSubstitution back shows the first line is satisfied, so  \n\n\\[\n(x,y,z)=(1,1,1). \\tag{6}\n\\]\n\nStep 7.  Computing the difference–triples.  \nFor each admissible triple we obtain  \n\n\\[\n\\begin{aligned}\n(1,1,1)     &\\;\\Longrightarrow\\; T=(0,0,0),\\\\\n(-1,-1,1)   &\\;\\Longrightarrow\\; T=(0,-2,2),\\\\\n(-1,1,-1)   &\\;\\Longrightarrow\\; T=(-2,2,0),\\\\\n(1,-1,-1)   &\\;\\Longrightarrow\\; T=(2,0,-2).\n\\end{aligned}\n\\]\n\nStep 8.  Exhaustion.  \nSteps 3–6 list all rational solutions of the system, so the four triples above exhaust all possible ordered difference–triples.\n\nAnswer.  The ordered difference–triple  \n\n\\[\nT=(x-y,\\;y-z,\\;z-x)\n\\]\n\ncan take exactly the four rational values  \n\n\\[\n(0,0,0),\\qquad(0,-2,2),\\qquad(-2,2,0),\\qquad(2,0,-2).\n\\]\n\nIn particular, $T$ assumes only finitely many values, as required.\n\n——————————————————————————————————————",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.543090",
        "was_fixed": false,
        "difficulty_analysis": "1.  Extra high-degree constraint.  \n   Beyond the original quadratic symmetry, the problem now imposes a cubic symmetric equation.  Handling (2) together with (1) forces the solver to juggle expressions of mixed degrees and to recognise how the quadratic factorisation in (3) interacts with the cubic factorisation in (5).\n\n2.  Finer case-work.  \n   In the original problem the contradiction \\( -3=0\\) disposed of the “all-distinct’’ case immediately.  Here that case must be eliminated by a linear-algebra argument, and the surviving “two equal’’ case can no longer be settled by a mere substitution; one must still satisfy the cubic relation, which eventually singles out a unique rational pair.\n\n3.  Necessity of number-theoretic reasoning.  \n   Equation \\(x^{2}+z^{2}=0\\) in ℚ demands recognising that the only rational solution is the trivial one, a small but essential appeal to rational points on the circle \\(X^{2}+Y^{2}=0\\).\n\n4.  Strict finiteness emerges only after combining different algebraic levels.  \n   While (1) alone allows infinitely many rational solutions with \\(x=y\\) and free parameter \\(x\\), the higher-degree condition (2) collapses this infinitude to a single point.  Understanding this subtle interaction is the crux of the enhanced problem and is absent in the original version.\n\nConsequently the new variant is substantially harder: it involves a mixed-degree system, demands sharper algebraic manipulation, and relies on an explicit argument about rational solutions of a quadratic form, all of which lie beyond the scope of the initial kernel problem."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}