{
  "index": "1960-A-1",
  "type": "NT",
  "tag": [
    "NT",
    "ALG"
  ],
  "difficulty": "",
  "question": "1. Let \\( n \\) be a given positive integer. How many solutions are there in ordered positive integer pairs \\( (x, y) \\) to the equation\n\\[\n\\frac{x y}{x+y}=n ? \\quad \\quad(\\text { page } 516)\n\\]",
  "solution": "Solution. The given equation is equivalent to\n\\[\n(x-n)(y-n)=n^{2}\n\\]\n\nEvidently, either both \\( x>n \\) and \\( y>n \\), or \\( x<n \\) and \\( y<n \\). There are no solutions satisfying both \\( 0<x<n, 0<y<n \\) because then \\( |x-n||y-n|<n \\cdot n=n^{2} \\). Hence we need only look for integral solutions of (1) with \\( x-n>0, y-n>0 \\). It is clear that there are as many solutions as there are ordered factorizations of \\( n^{2} \\) into two factors.\n\nIf the prime factorization of \\( n \\) is \\( p_{1}^{\\alpha_{1}} p_{2}{ }^{\\alpha_{2}} \\cdots p_{k}{ }^{\\alpha_{k}} \\), then\n\\[\nn^{2}=p_{1}{ }^{2 \\alpha_{1}} p_{2}{ }^{2 \\alpha_{2}} \\cdots p_{k}{ }^{2 \\alpha_{k}}\n\\]\nand the number of ordered factorizations of \\( n^{2} \\) is\n\\[\n\\left(2 \\alpha_{1}+1\\right)\\left(2 \\alpha_{2}+1\\right) \\cdots\\left(2 \\alpha_{k}+1\\right)\n\\]",
  "vars": [
    "x",
    "y"
  ],
  "params": [
    "n",
    "p_1",
    "p_2",
    "p_k",
    "k",
    "\\\\alpha_1",
    "\\\\alpha_2",
    "\\\\alpha_k"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "firstvar",
        "y": "secondvar",
        "n": "positint",
        "p_1": "primeone",
        "p_2": "primetwo",
        "p_k": "primekay",
        "k": "indexvar",
        "\\alpha_1": "expoone",
        "\\alpha_2": "expotwo",
        "\\alpha_k": "expokay"
      },
      "question": "1. Let \\( positint \\) be a given positive integer. How many solutions are there in ordered positive integer pairs \\( (firstvar, secondvar) \\) to the equation\n\\[\n\\frac{firstvar\\, secondvar}{firstvar+secondvar}=positint ? \\quad \\quad(\\text { page } 516)\n\\]",
      "solution": "Solution. The given equation is equivalent to\n\\[\n(firstvar-positint)(secondvar-positint)=positint^{2}\n\\]\n\nEvidently, either both \\( firstvar>positint \\) and \\( secondvar>positint \\), or \\( firstvar<positint \\) and \\( secondvar<positint \\). There are no solutions satisfying both \\( 0<firstvar<positint, 0<secondvar<positint \\) because then \\( |firstvar-positint||secondvar-positint|<positint \\cdot positint=positint^{2} \\). Hence we need only look for integral solutions of (1) with \\( firstvar-positint>0, secondvar-positint>0 \\). It is clear that there are as many solutions as there are ordered factorizations of \\( positint^{2} \\) into two factors.\n\nIf the prime factorization of \\( positint \\) is \\( primeone^{expoone} primetwo^{expotwo} \\cdots primekay^{expokay} \\), then\n\\[\npositint^{2}=primeone^{2\\,expoone} primetwo^{2\\,expotwo} \\cdots primekay^{2\\,expokay}\n\\]\nand the number of ordered factorizations of \\( positint^{2} \\) is\n\\[\n\\left(2 expoone+1\\right)\\left(2 expotwo+1\\right) \\cdots\\left(2 expokay+1\\right)\n\\]"
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "marshmallow",
        "y": "pineapple",
        "n": "envelope",
        "p_1": "waterfall",
        "p_2": "shipwreck",
        "p_k": "hedgehog",
        "k": "cinnamon",
        "\\alpha_1": "suitcase",
        "\\alpha_2": "doorknob",
        "\\alpha_k": "riverbank"
      },
      "question": "1. Let \\( envelope \\) be a given positive integer. How many solutions are there in ordered positive integer pairs \\( (marshmallow, pineapple) \\) to the equation\n\\[\n\\frac{marshmallow\\, pineapple}{marshmallow+pineapple}=envelope ? \\quad \\quad(\\text { page } 516)\n\\]\n",
      "solution": "Solution. The given equation is equivalent to\n\\[\n(marshmallow-envelope)(pineapple-envelope)=envelope^{2}\n\\]\n\nEvidently, either both \\( marshmallow>envelope \\) and \\( pineapple>envelope \\), or \\( marshmallow<envelope \\) and \\( pineapple<envelope \\). There are no solutions satisfying both \\( 0<marshmallow<envelope, 0<pineapple<envelope \\) because then \\( |marshmallow-envelope||pineapple-envelope|<envelope \\cdot envelope=envelope^{2} \\). Hence we need only look for integral solutions of (1) with \\( marshmallow-envelope>0, pineapple-envelope>0 \\). It is clear that there are as many solutions as there are ordered factorizations of \\( envelope^{2} \\) into two factors.\n\nIf the prime factorization of \\( envelope \\) is \\( waterfall^{suitcase} shipwreck^{doorknob} \\cdots hedgehog^{riverbank} \\), then\n\\[\nenvelope^{2}=waterfall^{2 suitcase} shipwreck^{2 doorknob} \\cdots hedgehog^{2 riverbank}\n\\]\nand the number of ordered factorizations of \\( envelope^{2} \\) is\n\\[\n\\left(2 suitcase+1\\right)\\left(2 doorknob+1\\right) \\cdots\\left(2 riverbank+1\\right)\n\\]\n"
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "knownvalue",
        "y": "fixedvalue",
        "n": "variableint",
        "p_1": "compositenumone",
        "p_2": "compositenumtwo",
        "p_k": "compositenumk",
        "k": "solitary",
        "\\alpha_1": "ultragammaone",
        "\\alpha_2": "ultragammatwo",
        "\\alpha_k": "ultragammak"
      },
      "question": "1. Let \\( variableint \\) be a given positive integer. How many solutions are there in ordered positive integer pairs \\( (knownvalue, fixedvalue) \\) to the equation\n\\[\n\\frac{knownvalue\\, fixedvalue}{knownvalue+fixedvalue}=variableint ? \\quad \\quad(\\text { page } 516)\n\\]",
      "solution": "Solution. The given equation is equivalent to\n\\[\n(knownvalue-variableint)(fixedvalue-variableint)=variableint^{2}\n\\]\n\nEvidently, either both \\( knownvalue>variableint \\) and \\( fixedvalue>variableint \\), or \\( knownvalue<variableint \\) and \\( fixedvalue<variableint \\). There are no solutions satisfying both \\( 0<knownvalue<variableint, 0<fixedvalue<variableint \\) because then \\( |knownvalue-variableint||fixedvalue-variableint|<variableint \\cdot variableint=variableint^{2} \\). Hence we need only look for integral solutions of (1) with \\( knownvalue-variableint>0, fixedvalue-variableint>0 \\). It is clear that there are as many solutions as there are ordered factorizations of \\( variableint^{2} \\) into two factors.\n\nIf the prime factorization of \\( variableint \\) is \\( compositenumone^{ultragammaone} compositenumtwo^{ultragammatwo} \\cdots compositenumk^{ultragammak} \\), then\n\\[\nvariableint^{2}=compositenumone^{2 ultragammaone} compositenumtwo^{2 ultragammatwo} \\cdots compositenumk^{2 ultragammak}\n\\]\nand the number of ordered factorizations of \\( variableint^{2} \\) is\n\\[\n\\left(2 ultragammaone+1\\right)\\left(2 ultragammatwo+1\\right) \\cdots\\left(2 ultragammak+1\\right)\n\\]"
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "y": "hjgrksla",
        "n": "fdmqieyz",
        "p_1": "blarjcvu",
        "p_2": "mktnswop",
        "p_k": "sdvplkhe",
        "k": "trmzoyun",
        "\\alpha_1": "vgeczhpa",
        "\\alpha_2": "qunxfody",
        "\\alpha_k": "wybsrtaj"
      },
      "question": "1. Let \\( fdmqieyz \\) be a given positive integer. How many solutions are there in ordered positive integer pairs \\( (qzxwvtnp, hjgrksla) \\) to the equation\n\\[\n\\frac{qzxwvtnp hjgrksla}{qzxwvtnp+hjgrksla}=fdmqieyz ? \\quad \\quad(\\text { page } 516)\n\\]\n",
      "solution": "Solution. The given equation is equivalent to\n\\[\n(qzxwvtnp-fdmqieyz)(hjgrksla-fdmqieyz)=fdmqieyz^{2}\n\\]\n\nEvidently, either both \\( qzxwvtnp>fdmqieyz \\) and \\( hjgrksla>fdmqieyz \\), or \\( qzxwvtnp<fdmqieyz \\) and \\( hjgrksla<fdmqieyz \\). There are no solutions satisfying both \\( 0<qzxwvtnp<fdmqieyz, 0<hjgrksla<fdmqieyz \\) because then \\( |qzxwvtnp-fdmqieyz||hjgrksla-fdmqieyz|<fdmqieyz \\cdot fdmqieyz=fdmqieyz^{2} \\). Hence we need only look for integral solutions of (1) with \\( qzxwvtnp-fdmqieyz>0, hjgrksla-fdmqieyz>0 \\). It is clear that there are as many solutions as there are ordered factorizations of \\( fdmqieyz^{2} \\) into two factors.\n\nIf the prime factorization of \\( fdmqieyz \\) is \\( blarjcvu^{vgeczhpa} mktnswop{ }^{qunxfody} \\cdots sdvplkhe{ }^{wybsrtaj} \\), then\n\\[\nfdmqieyz^{2}=blarjcvu{ }^{2 vgeczhpa} mktnswop{ }^{2 qunxfody} \\cdots sdvplkhe{ }^{2 wybsrtaj}\n\\]\nand the number of ordered factorizations of \\( fdmqieyz^{2} \\) is\n\\[\n\\left(2 vgeczhpa+1\\right)\\left(2 qunxfody+1\\right) \\cdots\\left(2 wybsrtaj+1\\right)\n\\]\n"
    },
    "kernel_variant": {
      "question": "Fix a positive integer $n$.  How many unordered pairs \\(\\{x,y\\}\\) of integers that are strictly larger than $n$ satisfy\n\\[\n\\frac{xy}{x+y}=n\\;?\n\\]",
      "solution": "Write the given equation as\n\nxy = n(x+y)\n⇒ (x−n)(y−n) = n².\n\nSince x>n and y>n, set a = x−n > 0, b = y−n > 0 to obtain\n\nab = n².\n\nIf n = ∏_{i=1}^k p_i^{α_i}, then n² = ∏_{i=1}^k p_i^{2α_i} has\n\nD = ∏_{i=1}^k (2α_i + 1)\n\npositive divisors.  These yield D ordered pairs (a,b), and the number of unordered pairs is\n\n(D + 1)/2 = ½(∏_{i=1}^k (2α_i + 1) + 1).\n\nTherefore the number of unordered integer solutions {x,y} with x,y>n is\n\n\ntheboxed{\\tfrac12\\Bigl(∏_{i=1}^k(2α_i+1)+1\\Bigr)}.",
      "_meta": {
        "core_steps": [
          "Clear the denominator: xy = n(x + y) ⇒ (x − n)(y − n) = n².",
          "Sign/size check: x, y must both be > n (the < n case gives |x − n||y − n| < n², impossible).",
          "Introduce a = x − n, b = y − n; then a · b = n² gives a bijection with factor pairs of n².",
          "Count ordered factor pairs of n² via prime-factor exponents: n = ∏ p_i^{α_i} ⇒ #pairs = ∏ (2α_i + 1)."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Treat pairs as ordered vs. unordered; the bijection with factor pairs still works, only the final count changes by a factor of 2 (except when a = b).",
            "original": "ordered"
          },
          "slot2": {
            "description": "Domain stipulation on (x, y); ‘positive integers’ could be replaced by ‘integers > n’ (the sign argument already forces this).",
            "original": "positive integer"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "calculation"
}