{
  "index": "1949-B-1",
  "type": "NT",
  "tag": [
    "NT",
    "ANA"
  ],
  "difficulty": "",
  "question": "1. Each rational number \\( p / q \\) ( \\( p, q \\) relatively prime positive integers) of the open interval \\( (0,1) \\) is covered by a closed interval of length \\( 1 / 2 q^{2} \\), whose center is at \\( p / q \\). Prove that \\( \\sqrt{2} / 2 \\) is not covered by any of the above closed intervals.",
  "solution": "Solution. The problem may be restated as follows:\nShow that\n\\[\n\\left|\\frac{\\sqrt{2}}{2}-\\frac{p}{q}\\right| \\leq \\frac{1}{4 q^{2}}\n\\]\nis impossible if \\( p \\) and \\( q \\) are integers, \\( 0<p<q \\).\nHence suppose \\( p \\) and \\( q \\) are integers such that \\( 0<p<q \\). Then\n\\[\n\\frac{\\sqrt{2}}{2}+\\frac{p}{q}<2\n\\]\n\nTherefore, if (1) holds, we have\n\\[\n\\left|\\frac{1}{2}-\\frac{p^{2}}{q^{2}}\\right|=\\left|\\frac{\\sqrt{2}}{2}-\\frac{p}{q}\\right|\\left|\\frac{\\sqrt{2}}{2}+\\frac{p}{q}\\right|<\\frac{1}{2 q^{2}}\n\\]\nwhence \\( \\left|q^{2}-2 p^{2}\\right|<1 \\). But \\( q^{2}-2 p^{2} \\) is an integer, so this implies that \\( q^{2}-2 p^{2}=0 \\). But this is impossible since \\( \\sqrt{2} \\) is irrational. Clearly (1) is impossible for integers \\( p, q \\) such that \\( p \\geq q>0 \\).\n\nRemarks. The hypothesis that \\( p \\) and \\( q \\) be relatively prime is unimportant.\n\nHurwitz (1891) proved the following theorem concerning the approximation of irrational numbers by rational numbers.\n\nFor any irrational number \\( \\alpha \\) there are infinitely many pairs of integers \\( p, q \\) such that\n\\[\n\\left|\\alpha-\\frac{p}{q}\\right|<\\frac{1}{\\sqrt{5} q^{2}}\n\\]\n\nOn the other hand, the inequality\n\\[\n\\left|\\frac{1+\\sqrt{5}}{2}-\\frac{p}{q}\\right|<\\frac{1}{k q^{2}}\n\\]\nhas only finitely many solutions if \\( k>\\sqrt{5} \\), so the constant appearing in Hurwitz' theorem is best possible. See Hardy and Wright, An Introduction to the Theory of Numbers, Oxford, 1938, p. 163.",
  "vars": [
    "p",
    "q"
  ],
  "params": [
    "\\\\alpha",
    "k"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "p": "numerator",
        "q": "denominator",
        "\\alpha": "alphavar",
        "k": "coefficient"
      },
      "question": "1. Each rational number \\( numerator / denominator \\) ( \\( numerator, denominator \\) relatively prime positive integers) of the open interval \\( (0,1) \\) is covered by a closed interval of length \\( 1 / 2 denominator^{2} \\), whose center is at \\( numerator / denominator \\). Prove that \\( \\sqrt{2} / 2 \\) is not covered by any of the above closed intervals.",
      "solution": "Solution. The problem may be restated as follows:\nShow that\n\\[\n\\left|\\frac{\\sqrt{2}}{2}-\\frac{numerator}{denominator}\\right| \\leq \\frac{1}{4 denominator^{2}}\n\\]\nis impossible if \\( numerator \\) and \\( denominator \\) are integers, \\( 0<numerator<denominator \\).\nHence suppose \\( numerator \\) and \\( denominator \\) are integers such that \\( 0<numerator<denominator \\). Then\n\\[\n\\frac{\\sqrt{2}}{2}+\\frac{numerator}{denominator}<2\n\\]\n\nTherefore, if (1) holds, we have\n\\[\n\\left|\\frac{1}{2}-\\frac{numerator^{2}}{denominator^{2}}\\right|=\\left|\\frac{\\sqrt{2}}{2}-\\frac{numerator}{denominator}\\right|\\left|\\frac{\\sqrt{2}}{2}+\\frac{numerator}{denominator}\\right|<\\frac{1}{2 denominator^{2}}\n\\]\nwhence \\( \\left|denominator^{2}-2 numerator^{2}\\right|<1 \\). But \\( denominator^{2}-2 numerator^{2} \\) is an integer, so this implies that \\( denominator^{2}-2 numerator^{2}=0 \\). But this is impossible since \\( \\sqrt{2} \\) is irrational. Clearly (1) is impossible for integers \\( numerator, denominator \\) such that \\( numerator \\geq denominator>0 \\).\n\nRemarks. The hypothesis that \\( numerator \\) and \\( denominator \\) be relatively prime is unimportant.\n\nHurwitz (1891) proved the following theorem concerning the approximation of irrational numbers by rational numbers.\n\nFor any irrational number \\( alphavar \\) there are infinitely many pairs of integers \\( numerator, denominator \\) such that\n\\[\n\\left|alphavar-\\frac{numerator}{denominator}\\right|<\\frac{1}{\\sqrt{5} denominator^{2}}\n\\]\n\nOn the other hand, the inequality\n\\[\n\\left|\\frac{1+\\sqrt{5}}{2}-\\frac{numerator}{denominator}\\right|<\\frac{1}{coefficient denominator^{2}}\n\\]\nhas only finitely many solutions if \\( coefficient>\\sqrt{5} \\), so the constant appearing in Hurwitz' theorem is best possible. See Hardy and Wright, An Introduction to the Theory of Numbers, Oxford, 1938, p. 163."
    },
    "descriptive_long_confusing": {
      "map": {
        "p": "dandelion",
        "q": "butterfly",
        "\\alpha": "sandcastle",
        "k": "marzipans"
      },
      "question": "Each rational number \\( dandelion / butterfly \\) ( \\( dandelion, butterfly \\) relatively prime positive integers) of the open interval \\( (0,1) \\) is covered by a closed interval of length \\( 1 / 2 butterfly^{2} \\), whose center is at \\( dandelion / butterfly \\). Prove that \\( \\sqrt{2} / 2 \\) is not covered by any of the above closed intervals.",
      "solution": "Solution. The problem may be restated as follows:\nShow that\n\\[\n\\left|\\frac{\\sqrt{2}}{2}-\\frac{dandelion}{butterfly}\\right| \\leq \\frac{1}{4 butterfly^{2}}\n\\]\nis impossible if \\( dandelion \\) and \\( butterfly \\) are integers, \\( 0<dandelion<butterfly \\).\nHence suppose \\( dandelion \\) and \\( butterfly \\) are integers such that \\( 0<dandelion<butterfly \\). Then\n\\[\n\\frac{\\sqrt{2}}{2}+\\frac{dandelion}{butterfly}<2\n\\]\n\nTherefore, if (1) holds, we have\n\\[\n\\left|\\frac{1}{2}-\\frac{dandelion^{2}}{butterfly^{2}}\\right|=\\left|\\frac{\\sqrt{2}}{2}-\\frac{dandelion}{butterfly}\\right|\\left|\\frac{\\sqrt{2}}{2}+\\frac{dandelion}{butterfly}\\right|<\\frac{1}{2 butterfly^{2}}\n\\]\nwhence \\( \\left|butterfly^{2}-2 dandelion^{2}\\right|<1 \\). But \\( butterfly^{2}-2 dandelion^{2} \\) is an integer, so this implies that \\( butterfly^{2}-2 dandelion^{2}=0 \\). But this is impossible since \\( \\sqrt{2} \\) is irrational. Clearly (1) is impossible for integers \\( dandelion, butterfly \\) such that \\( dandelion \\geq butterfly>0 \\).\n\nRemarks. The hypothesis that \\( dandelion \\) and \\( butterfly \\) be relatively prime is unimportant.\n\nHurwitz (1891) proved the following theorem concerning the approximation of irrational numbers by rational numbers.\n\nFor any irrational number \\( sandcastle \\) there are infinitely many pairs of integers \\( dandelion, butterfly \\) such that\n\\[\n\\left|sandcastle-\\frac{dandelion}{butterfly}\\right|<\\frac{1}{\\sqrt{5} butterfly^{2}}\n\\]\n\nOn the other hand, the inequality\n\\[\n\\left|\\frac{1+\\sqrt{5}}{2}-\\frac{dandelion}{butterfly}\\right|<\\frac{1}{marzipans butterfly^{2}}\n\\]\nhas only finitely many solutions if \\( marzipans>\\sqrt{5} \\), so the constant appearing in Hurwitz' theorem is best possible. See Hardy and Wright, An Introduction to the Theory of Numbers, Oxford, 1938, p. 163."
    },
    "descriptive_long_misleading": {
      "map": {
        "p": "nonprime",
        "q": "numerator",
        "\\\\alpha": "rational",
        "k": "variable"
      },
      "question": "1. Each rational number \\( nonprime / numerator \\) ( \\( nonprime, numerator \\) relatively prime positive integers) of the open interval \\( (0,1) \\) is covered by a closed interval of length \\( 1 / 2 numerator^{2} \\), whose center is at \\( nonprime / numerator \\). Prove that \\( \\sqrt{2} / 2 \\) is not covered by any of the above closed intervals.",
      "solution": "Solution. The problem may be restated as follows:\nShow that\n\\[\n\\left|\\frac{\\sqrt{2}}{2}-\\frac{nonprime}{numerator}\\right| \\leq \\frac{1}{4 numerator^{2}}\n\\]\nis impossible if \\( nonprime \\) and \\( numerator \\) are integers, \\( 0<nonprime<numerator \\).\nHence suppose \\( nonprime \\) and \\( numerator \\) are integers such that \\( 0<nonprime<numerator \\). Then\n\\[\n\\frac{\\sqrt{2}}{2}+\\frac{nonprime}{numerator}<2\n\\]\n\nTherefore, if (1) holds, we have\n\\[\n\\left|\\frac{1}{2}-\\frac{nonprime^{2}}{numerator^{2}}\\right|=\\left|\\frac{\\sqrt{2}}{2}-\\frac{nonprime}{numerator}\\right|\\left|\\frac{\\sqrt{2}}{2}+\\frac{nonprime}{numerator}\\right|<\\frac{1}{2 numerator^{2}}\n\\]\nwhence \\( \\left|numerator^{2}-2 nonprime^{2}\\right|<1 \\). But \\( numerator^{2}-2 nonprime^{2} \\) is an integer, so this implies that \\( numerator^{2}-2 nonprime^{2}=0 \\). But this is impossible since \\( \\sqrt{2} \\) is irrational. Clearly (1) is impossible for integers \\( nonprime, numerator \\) such that \\( nonprime \\geq numerator>0 \\).\n\nRemarks. The hypothesis that \\( nonprime \\) and \\( numerator \\) be relatively prime is unimportant.\n\nHurwitz (1891) proved the following theorem concerning the approximation of irrational numbers by rational numbers.\n\nFor any irrational number \\( rational \\) there are infinitely many pairs of integers \\( nonprime, numerator \\) such that\n\\[\n\\left|rational-\\frac{nonprime}{numerator}\\right|<\\frac{1}{\\sqrt{5} numerator^{2}}\n\\]\n\nOn the other hand, the inequality\n\\[\n\\left|\\frac{1+\\sqrt{5}}{2}-\\frac{nonprime}{numerator}\\right|<\\frac{1}{variable numerator^{2}}\n\\]\nhas only finitely many solutions if \\( variable>\\sqrt{5} \\), so the constant appearing in Hurwitz' theorem is best possible. See Hardy and Wright, An Introduction to the Theory of Numbers, Oxford, 1938, p. 163."
    },
    "garbled_string": {
      "map": {
        "p": "zqjwnfyl",
        "q": "hvpsrmta",
        "\\alpha": "njgqwdkc",
        "k": "qrstplmn"
      },
      "question": "1. Each rational number \\( zqjwnfyl / hvpsrmta \\) ( \\( zqjwnfyl, hvpsrmta \\) relatively prime positive integers) of the open interval \\( (0,1) \\) is covered by a closed interval of length \\( 1 / 2 hvpsrmta^{2} \\), whose center is at \\( zqjwnfyl / hvpsrmta \\). Prove that \\( \\sqrt{2} / 2 \\) is not covered by any of the above closed intervals.",
      "solution": "Solution. The problem may be restated as follows:\nShow that\n\\[\n\\left|\\frac{\\sqrt{2}}{2}-\\frac{zqjwnfyl}{hvpsrmta}\\right| \\leq \\frac{1}{4 hvpsrmta^{2}}\n\\]\nis impossible if \\( zqjwnfyl \\) and \\( hvpsrmta \\) are integers, \\( 0<zqjwnfyl<hvpsrmta \\).\nHence suppose \\( zqjwnfyl \\) and \\( hvpsrmta \\) are integers such that \\( 0<zqjwnfyl<hvpsrmta \\). Then\n\\[\n\\frac{\\sqrt{2}}{2}+\\frac{zqjwnfyl}{hvpsrmta}<2\n\\]\n\nTherefore, if (1) holds, we have\n\\[\n\\left|\\frac{1}{2}-\\frac{zqjwnfyl^{2}}{hvpsrmta^{2}}\\right|=\\left|\\frac{\\sqrt{2}}{2}-\\frac{zqjwnfyl}{hvpsrmta}\\right|\\left|\\frac{\\sqrt{2}}{2}+\\frac{zqjwnfyl}{hvpsrmta}\\right|<\\frac{1}{2 hvpsrmta^{2}}\n\\]\nwhence \\( \\left|hvpsrmta^{2}-2 zqjwnfyl^{2}\\right|<1 \\). But \\( hvpsrmta^{2}-2 zqjwnfyl^{2} \\) is an integer, so this implies that \\( hvpsrmta^{2}-2 zqjwnfyl^{2}=0 \\). But this is impossible since \\( \\sqrt{2} \\) is irrational. Clearly (1) is impossible for integers \\( zqjwnfyl, hvpsrmta \\) such that \\( zqjwnfyl \\geq hvpsrmta>0 \\).\n\nRemarks. The hypothesis that \\( zqjwnfyl \\) and \\( hvpsrmta \\) be relatively prime is unimportant.\n\nHurwitz (1891) proved the following theorem concerning the approximation of irrational numbers by rational numbers.\n\nFor any irrational number \\( njgqwdkc \\) there are infinitely many pairs of integers \\( zqjwnfyl, hvpsrmta \\) such that\n\\[\n\\left|njgqwdkc-\\frac{zqjwnfyl}{hvpsrmta}\\right|<\\frac{1}{\\sqrt{5} hvpsrmta^{2}}\n\\]\n\nOn the other hand, the inequality\n\\[\n\\left|\\frac{1+\\sqrt{5}}{2}-\\frac{zqjwnfyl}{hvpsrmta}\\right|<\\frac{1}{qrstplmn hvpsrmta^{2}}\n\\]\nhas only finitely many solutions if \\( qrstplmn>\\sqrt{5} \\), so the constant appearing in Hurwitz' theorem is best possible. See Hardy and Wright, An Introduction to the Theory of Numbers, Oxford, 1938, p. 163."
    },
    "kernel_variant": {
      "question": "Let\n\na = √3 / 3.\n\nFor every reduced rational number p / q ∈ (0 , 1) (0 < p < q, gcd(p , q) = 1) whose denominator is a multiple of 5, attach the open interval\n\n    I_{p/q} = ( p / q − 1 / (6 q²) ,  p / q + 1 / (6 q²) ).\n\nProve that a is contained in none of these intervals, i.e.\n\n    a ∉ ⋃_{ 0 < p < q , 5 | q } I_{p/q}.",
      "solution": "Assume, with the aim of deriving a contradiction, that there exist integers p , q with\n\n    0 < p < q,      5 | q,      and      √3/3 ∈ I_{p/q}.\n\nBy the definition of I_{p/q} this means\n\n    |√3/3 − p/q| < 1 / (6 q²).           (1)\n\nStep 1.  Eliminate the square root.\n\nBecause 0 < √3/3 , p / q < 1 we have\n\n    √3/3 + p / q < 2.                   (2)\n\nMultiplying (1) and (2) gives\n\n    | (√3/3)² − (p / q)² | < 2 · 1 / (6 q²) = 1 / (3 q²).        (3)\n\nSince (√3/3)² = 1 / 3, inequality (3) is\n\n    | 1 / 3 − p² / q² | < 1 / (3 q²).\n\nMultiplying by 3 q² we obtain the purely integral inequality\n\n    | q² − 3 p² | < 1.                                  (4)\n\nStep 2.  Contradictory conclusion.\n\nThe quantity q² − 3 p² is an integer.  By (4) its absolute value is strictly smaller than 1, hence it must be 0:\n\n    q² − 3 p² = 0  ⇒  q² = 3 p².                         (5)\n\nBut (5) implies q / p = √3, contradicting the well-known fact that √3 is irrational.  The contradiction originated from the assumption that √3/3 belongs to one of the intervals, therefore no such interval exists:\n\n    √3 / 3 ∉ ⋃_{ 0 < p < q , 5 | q } I_{p/q}.\n\nRemarks on the two additional hypotheses\n\n•  Openness of the intervals.  Inequality (1) is strict because the intervals are open; this yields the strict inequality in (4).  If the intervals were closed we would only have |q² − 3 p²| ≤ 1, and the Pell equations q² − 3 p² = ±1 do possess infinitely many integer solutions.  Hence openness is essential for the above short proof.\n\n•  The divisibility restriction 5 | q is not used in the argument, so the statement we have proved is in fact stronger than asked.  Alternatively, had the intervals been closed, the condition 5 | q would become crucial, because none of the solutions of q² − 3 p² = ±1 has a denominator divisible by 5.  Thus the problem setter ensured impossibility by requiring *either* open intervals *or* the factor 5 in the denominator; here we rely only on the former.",
      "_meta": {
        "core_steps": [
          "Assume a rational p/q satisfies |α − p/q| ≤ c/(2q²) (half-length of given interval).",
          "Use α+p/q < 2 (since α, p/q ∈ (0,1)) to get |α² − (p/q)²| < c/q².",
          "Rewrite as |q²·α² − p²| < c.",
          "Because the left side is an integer and 0 < c < 1, it must equal 0.",
          "This forces q²·α² = p², contradicting the irrationality of α."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "‘Relatively prime’ requirement on p and q (never used).",
            "original": "relatively prime"
          },
          "slot2": {
            "description": "Choice of ‘closed’ for the covering intervals (openness/closedness irrelevant).",
            "original": "closed"
          },
          "slot3": {
            "description": "Radicand under the square root defining α; any non-square integer works.",
            "original": "2 (in √2)"
          },
          "slot4": {
            "description": "Denominator scaling α = √2 / 2; any positive integer giving α < 1 and α² rational works.",
            "original": "2 (divisor of √2)"
          },
          "slot5": {
            "description": "Coefficient 1/2 in the interval length; any constant c with 0 < c < 1 keeps the argument.",
            "original": "1/2"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}