{
  "index": "2010-A-4",
  "type": "NT",
  "tag": [
    "NT",
    "ALG"
  ],
  "difficulty": "",
  "question": "Prove that for each positive integer $n$, the number\n$10^{10^{10^n}} + 10^{10^n} + 10^n - 1$\nis not prime.",
  "solution": "Put\n\\[\nN = 10^{10^{10^n}} + 10^{10^n} + 10^n - 1.\n\\]\nWrite $n = 2^m k$ with $m$ a nonnegative integer and $k$ a positive odd integer.\nFor any nonnegative integer $j$,\n\\[\n10^{2^m j} \\equiv (-1)^j \\pmod{10^{2^m} + 1}.\n\\]\nSince $10^n \\geq n \\geq 2^m \\geq m+1$, $10^n$ is divisible by $2^n$ and hence by $2^{m+1}$,\nand similarly $10^{10^n}$ is divisible by $2^{10^n}$ and hence by $2^{m+1}$. It follows that\n\\[\nN \\equiv 1 + 1 + (-1) + (-1) \\equiv 0 \\pmod{10^{2^m} + 1}.\n\\]\nSince $N \\geq 10^{10^n} > 10^n + 1 \\geq 10^{2^m} + 1$, it follows that $N$ is composite.",
  "vars": [
    "n",
    "N",
    "m",
    "k",
    "j"
  ],
  "params": [],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "n": "indexn",
        "N": "targetnum",
        "m": "powerm",
        "k": "oddpart",
        "j": "counter"
      },
      "question": "Prove that for each positive integer indexn, the number\n$10^{10^{10^{indexn}}} + 10^{10^{indexn}} + 10^{indexn} - 1$\nis not prime.",
      "solution": "Put\n\\[\ntargetnum = 10^{10^{10^{indexn}}} + 10^{10^{indexn}} + 10^{indexn} - 1.\n\\]\nWrite $indexn = 2^{powerm} oddpart$ with $powerm$ a nonnegative integer and $oddpart$ a positive odd integer.\nFor any nonnegative integer $counter$,\n\\[\n10^{2^{powerm} counter} \\equiv (-1)^{counter} \\pmod{10^{2^{powerm}} + 1}.\n\\]\nSince $10^{indexn} \\geq indexn \\geq 2^{powerm} \\geq powerm+1$, $10^{indexn}$ is divisible by $2^{indexn}$ and hence by $2^{powerm+1}$,\nand similarly $10^{10^{indexn}}$ is divisible by $2^{10^{indexn}}$ and hence by $2^{powerm+1}$. It follows that\n\\[\ntargetnum \\equiv 1 + 1 + (-1) + (-1) \\equiv 0 \\pmod{10^{2^{powerm}} + 1}.\n\\]\nSince $targetnum \\geq 10^{10^{indexn}} > 10^{indexn} + 1 \\geq 10^{2^{powerm}} + 1$, it follows that $targetnum$ is composite."
    },
    "descriptive_long_confusing": {
      "map": {
        "n": "pineapple",
        "N": "asteroid",
        "m": "marigold",
        "k": "telescope",
        "j": "waterfall"
      },
      "question": "Prove that for each positive integer $\\text{pineapple}$, the number\n$10^{10^{10^{\\text{pineapple}}}} + 10^{10^{\\text{pineapple}}} + 10^{\\text{pineapple}} - 1$\nis not prime.",
      "solution": "Put\n\\[\n\\text{asteroid} = 10^{10^{10^{\\text{pineapple}}}} + 10^{10^{\\text{pineapple}}} + 10^{\\text{pineapple}} - 1.\n\\]\nWrite $\\text{pineapple} = 2^{\\text{marigold}}\\,\\text{telescope}$ with $\\text{marigold}$ a nonnegative integer and $\\text{telescope}$ a positive odd integer.\nFor any nonnegative integer $\\text{waterfall}$,\n\\[\n10^{2^{\\text{marigold}}\\,\\text{waterfall}} \\equiv (-1)^{\\text{waterfall}} \\pmod{10^{2^{\\text{marigold}}} + 1}.\n\\]\nSince $10^{\\text{pineapple}} \\geq \\text{pineapple} \\geq 2^{\\text{marigold}} \\geq \\text{marigold}+1$, $10^{\\text{pineapple}}$ is divisible by $2^{\\text{pineapple}}$ and hence by $2^{\\text{marigold}+1}$, and similarly $10^{10^{\\text{pineapple}}}$ is divisible by $2^{10^{\\text{pineapple}}}$ and hence by $2^{\\text{marigold}+1}$. It follows that\n\\[\n\\text{asteroid} \\equiv 1 + 1 + (-1) + (-1) \\equiv 0 \\pmod{10^{2^{\\text{marigold}}} + 1}.\n\\]\nSince $\\text{asteroid} \\geq 10^{10^{\\text{pineapple}}} > 10^{\\text{pineapple}} + 1 \\geq 10^{2^{\\text{marigold}}} + 1$, it follows that $\\text{asteroid}$ is composite."
    },
    "descriptive_long_misleading": {
      "map": {
        "n": "negativestep",
        "N": "minusculevalue",
        "m": "negativeindex",
        "k": "evenfactor",
        "j": "negativecounter"
      },
      "question": "Prove that for each positive integer $negativestep$, the number\n$10^{10^{10^{negativestep}}} + 10^{10^{negativestep}} + 10^{negativestep} - 1$\nis not prime.",
      "solution": "Put\n\\[\nminusculevalue = 10^{10^{10^{negativestep}}} + 10^{10^{negativestep}} + 10^{negativestep} - 1.\n\\]\nWrite $negativestep = 2^{negativeindex} evenfactor$ with $negativeindex$ a nonnegative integer and $evenfactor$ a positive odd integer.\nFor any nonnegative integer $negativecounter$,\n\\[\n10^{2^{negativeindex} negativecounter} \\equiv (-1)^{negativecounter} \\pmod{10^{2^{negativeindex}} + 1}.\n\\]\nSince $10^{negativestep} \\geq negativestep \\geq 2^{negativeindex} \\geq negativeindex+1$, $10^{negativestep}$ is divisible by $2^{negativestep}$ and hence by $2^{negativeindex+1}$,\nand similarly $10^{10^{negativestep}}$ is divisible by $2^{10^{negativestep}}$ and hence by $2^{negativeindex+1}$. It follows that\n\\[\nminusculevalue \\equiv 1 + 1 + (-1) + (-1) \\equiv 0 \\pmod{10^{2^{negativeindex}} + 1}.\n\\]\nSince $minusculevalue \\geq 10^{10^{negativestep}} > 10^{negativestep} + 1 \\geq 10^{2^{negativeindex}} + 1$, it follows that $minusculevalue$ is composite."
    },
    "garbled_string": {
      "map": {
        "n": "qzxwvtnp",
        "N": "mbyplqez",
        "m": "vdhjcper",
        "k": "wmslqfzn",
        "j": "sbprtxew"
      },
      "question": "Prove that for each positive integer $qzxwvtnp$, the number\n$10^{10^{10^{qzxwvtnp}}} + 10^{10^{qzxwvtnp}} + 10^{qzxwvtnp} - 1$\nis not prime.",
      "solution": "Put\n\\[\nmbyplqez = 10^{10^{10^{qzxwvtnp}}} + 10^{10^{qzxwvtnp}} + 10^{qzxwvtnp} - 1.\n\\]\nWrite $qzxwvtnp = 2^{vdhjcper} wmslqfzn$ with $vdhjcper$ a nonnegative integer and $wmslqfzn$ a positive odd integer.\nFor any nonnegative integer $sbprtxew$,\n\\[\n10^{2^{vdhjcper} sbprtxew} \\equiv (-1)^{sbprtxew} \\pmod{10^{2^{vdhjcper}} + 1}.\n\\]\nSince $10^{qzxwvtnp} \\geq qzxwvtnp \\geq 2^{vdhjcper} \\geq vdhjcper+1$, $10^{qzxwvtnp}$ is divisible by $2^{qzxwvtnp}$ and hence by $2^{vdhjcper+1}$,\nand similarly $10^{10^{qzxwvtnp}}$ is divisible by $2^{10^{qzxwvtnp}}$ and hence by $2^{vdhjcper+1}$. It follows that\n\\[\nmbyplqez \\equiv 1 + 1 + (-1) + (-1) \\equiv 0 \\pmod{10^{2^{vdhjcper}} + 1}.\n\\]\nSince $mbyplqez \\geq 10^{10^{qzxwvtnp}} > 10^{qzxwvtnp} + 1 \\geq 10^{2^{vdhjcper}} + 1$, it follows that $mbyplqez$ is composite."
    },
    "kernel_variant": {
      "question": "Let n be a positive even integer.  Write  \n\n  n = 2^{\\alpha}\\,3^{\\beta}\\,k  with α ≥ 1, β ≥ 0, gcd(k,6)=1.  (★)\n\nDefine  \n\n  N(n)=6^{6^{6^{\\,n}}}+6^{6^{\\,n}}+6^{\\,n}-6^{3n}-6^{2n}-1.  \n\na) Prove simultaneously that  \n\n  6^{2^{\\alpha}}+1  divides  N(n) and 6^{3^{\\beta}}+1  divides  N(n).  \n\nb) Show that the two divisors in part a are coprime.  \n\nConsequently N(n) possesses at least two distinct non-trivial coprime divisors and is therefore composite for every even positive integer n.",
      "solution": "For brevity denote  \n\n  P := 6^{2^{\\alpha}}+1,  Q := 6^{3^{\\beta}}+1.  (1)\n\n1.  A basic congruence rule  \nLet M be a positive integer.  Since 6^{M}≡−1 (mod 6^{M}+1), for every integer t we have  \n\n  6^{Mt} ≡ (−1)^{\\,t} (mod 6^{M}+1).  (2)\n\nFormula (2) will repeatedly be used with M = 2^{\\alpha} and M = 3^{\\beta}.\n\n--------------------------------------------------------------------\n2.  Divisibility by P = 6^{2^{\\alpha}}+1  \nBecause α ≥ 1, the modulus M = 2^{\\alpha} is even.  We first determine, for each of the exponents that occur in N(n), whether the quotient by 2^{\\alpha} is even or odd.\n\nThe factorisation n = 2^{\\alpha}3^{\\beta}k with k odd implies  \n\n  6^{n} = (2·3)^{\\,n} contains at least 2^{n} as a power of two.  \n  Since n ≥ 2^{\\alpha}, we have n ≥ α+1 and hence 2^{\\alpha+1} | 2^{n} ⊂ 6^{n}.  \n\nTherefore 2^{\\alpha+1} divides 6^{n} and, a fortiori, it divides 6^{6^{n}}.  Collecting the information we get\n\n–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––\nTerm     Exponent e  e / 2^{\\alpha}  Parity of the quotient  \n6^{6^{6^{n}}}  e = 6^{6^{n}}  divisible by 2^{\\alpha+1}  even  \n6^{6^{n}}   e = 6^{n}   divisible by 2^{\\alpha+1}  even  \n6^{n}    e = n     = 3^{\\beta}k       odd (k odd)  \n6^{3n}    e = 3n    = 3·3^{\\beta}k     odd  \n6^{2n}    e = 2n    = 2·3^{\\beta}k     even  \n6^{0}=1    –       –        –  \n\nApplying (2) with M = 2^{\\alpha} we obtain  \n\n  6^{6^{6^{n}}} ≡  1 (mod P)  \n  6^{6^{n}}   ≡  1 (mod P)  \n  6^{n}      ≡ −1 (mod P)  \n  6^{3n}     ≡ −1 (mod P)  \n  6^{2n}     ≡  1 (mod P).  \n\nConsequently  \n\n  N(n) ≡ 1+1−1−(−1)−1−1 = 0 (mod P),  \n\nand therefore P | N(n).\n\n--------------------------------------------------------------------\n3.  Divisibility by Q = 6^{3^{\\beta}}+1  \nNow put M = 3^{\\beta}.  Because n=2^{\\alpha}3^{\\beta}k, we have\n\n  6^{n} = 6^{\\,2^{\\alpha}3^{\\beta}k} = (6^{3^{\\beta}})^{2^{\\alpha}k},  \n\nshowing that 3^{\\beta} divides 6^{n}; consequently it also divides 6^{6^{n}} and 6^{6^{6^{n}}}.  A parity table analogous to the previous one is\n\n–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––\nTerm     Exponent e  e / 3^{\\beta}  Parity of the quotient  \n6^{6^{6^{n}}}  e = 6^{6^{n}}  divisible by 3^{\\beta+1}  even  \n6^{6^{n}}   e = 6^{n}   divisible by 3^{\\beta+1}  even  \n6^{n}    e = n     = 2^{\\alpha}k     even (α ≥ 1)  \n6^{3n}    e = 3n    = 3·2^{\\alpha}k   even  \n6^{2n}    e = 2n    = 2·2^{\\alpha}k   even  \n6^{0}=1    –       –        –  \n\nUsing (2) with M = 3^{\\beta} we therefore get  \n\n  6^{6^{6^{n}}} ≡  1 (mod Q)  \n  6^{6^{n}}   ≡  1 (mod Q)  \n  6^{n}      ≡  1 (mod Q)  \n  6^{3n}     ≡  1 (mod Q)  \n  6^{2n}     ≡  1 (mod Q).  \n\nHence  \n\n  N(n) ≡ 1+1+1−1−1−1 = 0 (mod Q),  \n\nso Q | N(n).\n\n--------------------------------------------------------------------\n4.  Coprimality of P and Q  \nIt remains to show gcd(P,Q)=1.  Set  \n\n  m = 2^{\\alpha}, n = 3^{\\beta}.  \n\nBecause gcd(m,n)=1 and m is even while n is odd, we invoke the following lemma.\n\nLemma.  For any even base a and coprime positive integers r,s with at least one of r,s even,  \n  gcd(a^{r}+1, a^{s}+1)=1.  \n\nProof.  Let d = gcd(a^{r}+1, a^{s}+1).  Any common divisor d > 1 must be odd (each summand is odd).  Working modulo d, we have  \n\n  a^{r} ≡ −1 ≡ a^{s}.  \n\nHence a^{r−s} ≡ 1 (mod d).  Because gcd(r,s)=1, there exist integers u,v with ur−vs=1; multiplying the last congruence by suitable powers of a gives a ≡ 1 (mod d).  Substituting back yields 2 ≡ 0 (mod d), impossible for odd d.  Therefore d = 1. ∎\n\nApplying the lemma with a = 6, r = m (even) and s = n (odd) gives  \n\n  gcd(P,Q)=1.  (3)\n\n--------------------------------------------------------------------\n5.  Non-triviality and conclusion  \nBoth P and Q exceed 1, and clearly P,Q < N(n).  \nHence N(n) possesses two distinct non-trivial coprime divisors; therefore N(n) is composite for every even positive integer n satisfying (★).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.817099",
        "was_fixed": false,
        "difficulty_analysis": "•  Multiple prime exponents: n is decomposed simultaneously with respect to 2 and 3, forcing the solver to keep track of two independent exponent lattices.  \n•  Two independent congruences: one has to manufacture an expression whose residues are 0 modulo both 6^{2^{\\alpha}}+1 and 6^{3^{\\beta}}+1.  Ensuring cancellation in each modulus—while the parities differ—requires finer control of the signs in the six-term combination.  \n•  Coprimality argument: after exhibiting two divisors one still must show they are essentially coprime, otherwise no conclusion about compositeness could be drawn.  \n•  Sign-parity analysis: the proof hinges on careful parity studies of large exponents modulo 2 and 3, far subtler than the single-parity check in the original problem.  \n•  Overall length: the solution demands two separate applications of cyclotomic-style arguments, additional parity cases, and a final number-theoretic gcd check, all of which are absent from the original kernel variant.  \n\nThese layers of extra structure and casework make the enhanced variant substantially harder than both the original problem and the first kernel version."
      }
    },
    "original_kernel_variant": {
      "question": "Let n be a positive even integer.  Write  \n\n  n = 2^{\\alpha}\\,3^{\\beta}\\,k  with α ≥ 1, β ≥ 0, gcd(k,6)=1.  (★)\n\nDefine  \n\n  N(n)=6^{6^{6^{\\,n}}}+6^{6^{\\,n}}+6^{\\,n}-6^{3n}-6^{2n}-1.  \n\na) Prove simultaneously that  \n\n  6^{2^{\\alpha}}+1  divides  N(n) and 6^{3^{\\beta}}+1  divides  N(n).  \n\nb) Show that the two divisors in part a are coprime.  \n\nConsequently N(n) possesses at least two distinct non-trivial coprime divisors and is therefore composite for every even positive integer n.",
      "solution": "For brevity denote  \n\n  P := 6^{2^{\\alpha}}+1,  Q := 6^{3^{\\beta}}+1.  (1)\n\n1.  A basic congruence rule  \nLet M be a positive integer.  Since 6^{M}≡−1 (mod 6^{M}+1), for every integer t we have  \n\n  6^{Mt} ≡ (−1)^{\\,t} (mod 6^{M}+1).  (2)\n\nFormula (2) will repeatedly be used with M = 2^{\\alpha} and M = 3^{\\beta}.\n\n--------------------------------------------------------------------\n2.  Divisibility by P = 6^{2^{\\alpha}}+1  \nBecause α ≥ 1, the modulus M = 2^{\\alpha} is even.  We first determine, for each of the exponents that occur in N(n), whether the quotient by 2^{\\alpha} is even or odd.\n\nThe factorisation n = 2^{\\alpha}3^{\\beta}k with k odd implies  \n\n  6^{n} = (2·3)^{\\,n} contains at least 2^{n} as a power of two.  \n  Since n ≥ 2^{\\alpha}, we have n ≥ α+1 and hence 2^{\\alpha+1} | 2^{n} ⊂ 6^{n}.  \n\nTherefore 2^{\\alpha+1} divides 6^{n} and, a fortiori, it divides 6^{6^{n}}.  Collecting the information we get\n\n–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––\nTerm     Exponent e  e / 2^{\\alpha}  Parity of the quotient  \n6^{6^{6^{n}}}  e = 6^{6^{n}}  divisible by 2^{\\alpha+1}  even  \n6^{6^{n}}   e = 6^{n}   divisible by 2^{\\alpha+1}  even  \n6^{n}    e = n     = 3^{\\beta}k       odd (k odd)  \n6^{3n}    e = 3n    = 3·3^{\\beta}k     odd  \n6^{2n}    e = 2n    = 2·3^{\\beta}k     even  \n6^{0}=1    –       –        –  \n\nApplying (2) with M = 2^{\\alpha} we obtain  \n\n  6^{6^{6^{n}}} ≡  1 (mod P)  \n  6^{6^{n}}   ≡  1 (mod P)  \n  6^{n}      ≡ −1 (mod P)  \n  6^{3n}     ≡ −1 (mod P)  \n  6^{2n}     ≡  1 (mod P).  \n\nConsequently  \n\n  N(n) ≡ 1+1−1−(−1)−1−1 = 0 (mod P),  \n\nand therefore P | N(n).\n\n--------------------------------------------------------------------\n3.  Divisibility by Q = 6^{3^{\\beta}}+1  \nNow put M = 3^{\\beta}.  Because n=2^{\\alpha}3^{\\beta}k, we have\n\n  6^{n} = 6^{\\,2^{\\alpha}3^{\\beta}k} = (6^{3^{\\beta}})^{2^{\\alpha}k},  \n\nshowing that 3^{\\beta} divides 6^{n}; consequently it also divides 6^{6^{n}} and 6^{6^{6^{n}}}.  A parity table analogous to the previous one is\n\n–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––\nTerm     Exponent e  e / 3^{\\beta}  Parity of the quotient  \n6^{6^{6^{n}}}  e = 6^{6^{n}}  divisible by 3^{\\beta+1}  even  \n6^{6^{n}}   e = 6^{n}   divisible by 3^{\\beta+1}  even  \n6^{n}    e = n     = 2^{\\alpha}k     even (α ≥ 1)  \n6^{3n}    e = 3n    = 3·2^{\\alpha}k   even  \n6^{2n}    e = 2n    = 2·2^{\\alpha}k   even  \n6^{0}=1    –       –        –  \n\nUsing (2) with M = 3^{\\beta} we therefore get  \n\n  6^{6^{6^{n}}} ≡  1 (mod Q)  \n  6^{6^{n}}   ≡  1 (mod Q)  \n  6^{n}      ≡  1 (mod Q)  \n  6^{3n}     ≡  1 (mod Q)  \n  6^{2n}     ≡  1 (mod Q).  \n\nHence  \n\n  N(n) ≡ 1+1+1−1−1−1 = 0 (mod Q),  \n\nso Q | N(n).\n\n--------------------------------------------------------------------\n4.  Coprimality of P and Q  \nIt remains to show gcd(P,Q)=1.  Set  \n\n  m = 2^{\\alpha}, n = 3^{\\beta}.  \n\nBecause gcd(m,n)=1 and m is even while n is odd, we invoke the following lemma.\n\nLemma.  For any even base a and coprime positive integers r,s with at least one of r,s even,  \n  gcd(a^{r}+1, a^{s}+1)=1.  \n\nProof.  Let d = gcd(a^{r}+1, a^{s}+1).  Any common divisor d > 1 must be odd (each summand is odd).  Working modulo d, we have  \n\n  a^{r} ≡ −1 ≡ a^{s}.  \n\nHence a^{r−s} ≡ 1 (mod d).  Because gcd(r,s)=1, there exist integers u,v with ur−vs=1; multiplying the last congruence by suitable powers of a gives a ≡ 1 (mod d).  Substituting back yields 2 ≡ 0 (mod d), impossible for odd d.  Therefore d = 1. ∎\n\nApplying the lemma with a = 6, r = m (even) and s = n (odd) gives  \n\n  gcd(P,Q)=1.  (3)\n\n--------------------------------------------------------------------\n5.  Non-triviality and conclusion  \nBoth P and Q exceed 1, and clearly P,Q < N(n).  \nHence N(n) possesses two distinct non-trivial coprime divisors; therefore N(n) is composite for every even positive integer n satisfying (★).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.625431",
        "was_fixed": false,
        "difficulty_analysis": "•  Multiple prime exponents: n is decomposed simultaneously with respect to 2 and 3, forcing the solver to keep track of two independent exponent lattices.  \n•  Two independent congruences: one has to manufacture an expression whose residues are 0 modulo both 6^{2^{\\alpha}}+1 and 6^{3^{\\beta}}+1.  Ensuring cancellation in each modulus—while the parities differ—requires finer control of the signs in the six-term combination.  \n•  Coprimality argument: after exhibiting two divisors one still must show they are essentially coprime, otherwise no conclusion about compositeness could be drawn.  \n•  Sign-parity analysis: the proof hinges on careful parity studies of large exponents modulo 2 and 3, far subtler than the single-parity check in the original problem.  \n•  Overall length: the solution demands two separate applications of cyclotomic-style arguments, additional parity cases, and a final number-theoretic gcd check, all of which are absent from the original kernel variant.  \n\nThese layers of extra structure and casework make the enhanced variant substantially harder than both the original problem and the first kernel version."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}