1999 AIME 第 7 题

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7.

10001000 个开关,每个开关有四个位置,记为 AABBCC, 和 DD。 任意开关改变位置时, 只会从 AABB, 从 BBCC, 从 CCDD, 或从 DDAA。 初始时每个开关都在位置 AA。 这些开关分别标有 10001000 个不同的整数 2x3y5z2^x 3^y 5^z, 其中 xxyy, 和 zz 分别取 0,1,,90, 1, \ldots, 9。 在一个 10001000 步过程的第 ii 步, 第 ii 个开关前进一档,同时所有标签能整除第 ii 个开关标签的其他开关也前进一档。 完成第 10001000 步后,有多少个开关会在位置 AA

There is a set of 10001000 switches, each of which has four positions, called A,A, B,B, C,C, and D.D. When the position of any switch changes, it is only from AA to B,B, from BB to C,C, from CC to D,D, or from DD to A.A. Initially each switch is in position A.A. The switches are labeled with the 10001000 different integers 2x3y5z,2^x 3^y 5^z, where x,x, y,y, and zz take on the values 0,1,,9.0, 1, \ldots, 9. At step ii of a 10001000-step process, the iith switch is advanced one step, and so are all the other switches whose labels divide the label on the iith switch. After step 10001000 has been completed, how many switches will be in position A?A?

答案:650
知识点:质因数分解模运算补集计数
难度评级:2650
解答:

标签为 dd 的开关,正好会在每个第 ii 步标签为 dd 的倍数时前进一次。标签中 2x3y5z2^x 3^y 5^z 的倍数是所有满足 xx9x \le x' \le 9 等条件的 2x3y5z2^{x'} 3^{y'} 5^{z'},所以这个开关会前进 (10x)(10y)(10z)(10 - x)(10 - y)(10 - z) 次。它回到位置 AA 当且仅当这个次数是 44 的倍数。

a=10xa = 10 - xb=10yb = 10 - yc=10zc = 10 - z,每个都从 111010 取值。我们计算 abcabc 44 整除的三元组:要么三者全为奇数,要么恰有一个是偶数但不被 44 整除。在 1,,101, \ldots, 10 中有 55 个奇数,以及 33 个数(2,6,102, 6, 10)是奇数的两倍。第一类有 53=1255^3 = 125 个,第二类有 3352=2253 \cdot 3 \cdot 5^2 = 225 个,共 350350 个。

因此 1000350=6501000 - 350 = 650 个开关最终在位置 AA

The switch labeled dd is advanced exactly once for each step ii whose label is a multiple of d.d. The multiples of 2x3y5z2^x 3^y 5^z among the labels are the 2x3y5z2^{x'} 3^{y'} 5^{z'} with xx9,x \le x' \le 9, etc., so that switch advances (10x)(10y)(10z)(10 - x)(10 - y)(10 - z) times. It returns to position AA exactly when this count is a multiple of 4.4.

Write a=10x,a = 10 - x, b=10y,b = 10 - y, c=10z,c = 10 - z, each ranging over 11 through 10.10. We count the triples where abcabc is not divisible by 4:4: either all three are odd, or exactly one is even but not divisible by 4.4. Among 1,,101, \ldots, 10 there are 55 odd values and 33 values (2,6,102, 6, 10) that are twice an odd number. That gives 53=1255^3 = 125 triples of the first kind and 3352=2253 \cdot 3 \cdot 5^2 = 225 of the second, or 350350 in all.

Therefore 1000350=6501000 - 350 = 650 switches end in position A.A.

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