2022 AIME I 第 13 题

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13.

SS 为所有能表示成循环小数 0.abcd0.\overline{abcd} 的有理数的集合,其中数字 aabbccdd 中至少有一个非零。将 SS 中的数写成最简分数时,设 NN 为可能出现的不同分子个数。例如,44410410 都会被计入 SS 中的数所产生的这些不同分子中,因为 0.3636=4110.\overline{3636} = \frac{4}{11},且 0.1230=41033330.\overline{1230} = \frac{410}{3333}。求 NN 除以 10001000 的余数。

Let SS be the set of all rational numbers that can be expressed as a repeating decimal in the form 0.abcd,0.\overline{abcd}, where at least one of the digits a,a, b,b, c,c, or dd is nonzero. Let NN be the number of distinct numerators obtained when numbers in SS are written as fractions in lowest terms. For example, both 44 and 410410 are counted among the distinct numerators for numbers in SS because 0.3636=4110.\overline{3636} = \frac{4}{11} and 0.1230=4103333.0.\overline{1230} = \frac{410}{3333}. Find the remainder when NN is divided by 1000.1000.

答案:392
知识点:循环小数欧拉函数分类讨论
难度评级:3160
解答:

SS 中每个元素都等于 k9999\frac{k}{9999},其中 1k99991 \le k \le 9999,且 9999=32111019999 = 3^2 \cdot 11 \cdot 101。化为最简形式后为 mD\frac{m}{D},其中 D9999D \mid 9999mDm \le D、且 gcd(m,D)=1\gcd(m, D) = 1;反过来,任何这样的 mD\frac{m}{D} 都可由 k=m9999Dk = m \cdot \frac{9999}{D}。 得到。所以 NN 计数的是那些不超过并且与 99999999 的某个因数 DD 互质的整数 mm

按质数 3,11,1013, 11, 101 中哪些整除 mm 来分类,每次都使用与 mm。 互质的最大因数 DD。 如果 gcd(m,9999)=1\gcd(m, 9999) = 1,取 D=9999D = 9999:这样的 mmφ(9999)=6000\varphi(9999) = 6000 个。如果只有 3m3 \mid m,取 D=11101=1111D = 11 \cdot 101 = 1111:不超过 11111111 且避开 111110110133 的倍数有 370333=334370 - 33 - 3 = 334 个。如果只有 11m11 \mid m,取 D=9101=909D = 9 \cdot 101 = 909:得到 8227=5582 - 27 = 55 个。如果只有 101m101 \mid m,则 D=99<101D = 99 \lt 101,没有可行值。 如果 33m33 \mid m101m101 \nmid m,取 D=101D = 101:数值 33,66,9933, 66, 99 再给出 33 个;而任何被 31013 \cdot 1011110111 \cdot 101 整除的 mm 都会要求 D11D \le 11,不可能。

因此 N=6000+334+55+3N = 6000 + 334 + 55 + 3 =6392= 6392,除以 10001000 的余数为 392392

Every element of SS equals k9999\frac{k}{9999} for some 1k9999,1 \le k \le 9999, where 9999=3211101.9999 = 3^2 \cdot 11 \cdot 101. In lowest terms this is mD\frac{m}{D} where D9999,D \mid 9999, mD,m \le D, and gcd(m,D)=1;\gcd(m, D) = 1; conversely any such mD\frac{m}{D} arises from k=m9999D.k = m \cdot \frac{9999}{D}. So NN counts the integers mm that are at most, and coprime to, some divisor DD of 9999.9999.

Classify mm by which of the primes 3,11,1013, 11, 101 divide it, always using the largest divisor DD coprime to m.m. If gcd(m,9999)=1,\gcd(m, 9999) = 1, take D=9999:D = 9999: there are φ(9999)=6000\varphi(9999) = 6000 such m.m. If 3m3 \mid m only, take D=11101=1111:D = 11 \cdot 101 = 1111: multiples of 33 up to 11111111 avoiding 1111 and 101101 number 370333=334.370 - 33 - 3 = 334. If 11m11 \mid m only, take D=9101=909:D = 9 \cdot 101 = 909: that gives 8227=55.82 - 27 = 55. If 101m101 \mid m only, then D=99<101D = 99 \lt 101 admits none. If 33m33 \mid m but 101m,101 \nmid m, take D=101:D = 101: the values 33,66,9933, 66, 99 give 33 more, and any mm divisible by 31013 \cdot 101 or 1110111 \cdot 101 would need D11,D \le 11, which is impossible.

Therefore N=6000+334+55+3N = 6000 + 334 + 55 + 3 =6392,= 6392, and the remainder modulo 10001000 is 392.392.

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