2026 AIME I 第 2 题

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2.

1010 进制表示中不含数字零、各位数字和为 1313 的正整数回文数的个数。例如,4212442124 满足这些性质。回顾一下,回文数是从左到右读和从右到左读表示都相同的数。

Find the number of positive integer palindromes written in base 10,10, with no zero digits, and whose digits add up to 13.13. For example, 4212442124 has these properties. Recall that a palindrome is a number whose representation reads the same from left to right as from right to left.

答案:62
知识点:回文数数字分拆与有序分拆奇偶性
难度评级:2110
解答:

偶数位回文数的每个数字都会成镜像对出现,所以数字和为偶数。由于 1313 是奇数, 回文数必须有奇数位;若 mm 是中间数字,则剩余数字和 13m13 - m 平均分到两侧, 所以 mm 为奇数。一位回文数需要 m=13m = 13,这不可能。

这个回文数由中间数字 mm 和中心左侧的数字块决定:这是一个非空的非零数字串,数字和为 s=13m2s = \frac{13 - m}{2}。当 m=1,3,5,7,9m = 1, 3, 5, 7, 9 时,分别有 s=6,5,4,3,2s = 6, 5, 4, 3, 2。由于 s6s \le 6,这样的数字串中每个数字自动不超过 99, 所以字符串个数就是 ss 的正整数拆分成有序部分的个数,即 2s12^{s-1} (在 s1s - 1 个单位间隙中,每个间隙选择断开或不断开)。

总数为 25+24+23+22+21=32+16+8+4+2=62. \begin{aligned} &2^{5} + 2^{4} + 2^{3} + 2^{2} + 2^{1} \\ &= 32 + 16 + 8 + 4 + 2 \\ &= 62. \end{aligned}

A palindrome with an even number of digits has each digit appearing in a mirrored pair, so its digit sum is even. Since 1313 is odd, the palindrome has an odd number of digits, and if mm is the middle digit, the rest of the digit sum 13m13 - m is split evenly between the two halves, so mm is odd. A one-digit palindrome would need m=13,m = 13, which is impossible.

The palindrome is determined by its middle digit mm and the block of digits to the left of center: a nonempty string of nonzero digits with sum s=13m2.s = \frac{13 - m}{2}. For m=1,3,5,7,9m = 1, 3, 5, 7, 9 we get s=6,5,4,3,2.s = 6, 5, 4, 3, 2. Since s6,s \le 6, every digit of such a string is automatically at most 9,9, so the number of strings is the number of compositions of s,s, which is 2s12^{s-1} (each of the s1s - 1 gaps between units is either a break or not).

The total is 25+24+23+22+21=32+16+8+4+2=62. \begin{aligned} &2^{5} + 2^{4} + 2^{3} + 2^{2} + 2^{1} \\ &= 32 + 16 + 8 + 4 + 2 \\ &= 62. \end{aligned}

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