2026 AIME I 第 4 题

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4.

求不超过 100100 的整数中,有多少个可以写成 a+b+aba + b + ab,其中 aabb 是两个不同的正整数。

Find the number of integers less than or equal to 100100 that are equal to a+b+aba + b + ab for some choice of distinct positive integers aa and b.b.

答案:70
知识点:西蒙最爱的因式分解技巧补集计数质数
难度评级:2300
解答:

因为 a+b+ab=(a+1)(b+1)1a + b + ab = (a+1)(b+1) - 1,整数 nn 可表示当且仅当 n+1=xyn + 1 = xy,其中 x=a+1x = a + 1y=b+1y = b + 1 是不同整数且都至少为 22。 所以我们统计集合 {2,3,,101}\{2, 3, \ldots, 101\} 中能如此分解的整数 n+1n + 1

质数不能分解成两个都至少为 22 的因数;质数平方 p2p^2 只能这样分解为 ppp \cdot p,但这不允许。其余每个合数 MM 都可行:若 pp 是它的最小质因数, 则 M=pMpM = p \cdot \frac{M}{p},且由于 M>p2M \gt p^2Mp>p\frac{M}{p} \gt p。 在 {2,,101}\{2, \ldots, 101\} 中有 2626 个质数(小于 1001002525 个质数, 再加上 101101)和 44 个质数平方(449925254949)。

个数为 100264=70100 - 26 - 4 = 70

Since a+b+ab=(a+1)(b+1)1,a + b + ab = (a+1)(b+1) - 1, an integer nn is representable exactly when n+1=xyn + 1 = xy for distinct integers x=a+1x = a + 1 and y=b+1y = b + 1 that are each at least 2.2. So we count integers n+1n + 1 in {2,3,,101}\{2, 3, \ldots, 101\} that admit such a factorization.

A prime has no factorization into two factors that are both at least 2,2, and the square of a prime p2p^2 factors that way only as pp,p \cdot p, which is not allowed. Every other composite MM works: if pp is its smallest prime factor, then M=pMpM = p \cdot \frac{M}{p} with Mp>p\frac{M}{p} \gt p since M>p2.M \gt p^2. In {2,,101}\{2, \ldots, 101\} there are 2626 primes (the 2525 primes below 100,100, together with 101101) and 44 prime squares (4,4, 9,9, 25,25, 4949).

The count is 100264=70.100 - 26 - 4 = 70.

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