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Number theory Difficulty 2.8 Junior Find the answer

How many positive integers n20000n \leq 20000 have the properties that 2n2n has 64 positive divisors including 1 and 2n2n, and 5n5n has 60 positive divisors including 1 and 5n5n?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Suppose n=2r5sp3a3p4a4pkakn=2^{r}5^{s}p_{3}^{a_{3}}p_{4}^{a_{4}}\cdots p_{k}^{a_{k}}. Since 2n2n has 64 divisors and 5n5n has 60 divisors, (r+2)(s+1)(a3+1)(a4+1)(ak+1)=64(r+2)(s+1)\left(a_{3}+1\right)\left(a_{4}+1\right)\cdots\left(a_{k}+1\right)=64 and (r+1)(s+2)(a3+1)(a4+1)(ak+1)=60(r+1)(s+2)\left(a_{3}+1\right)\left(a_{4}+1\right)\cdots\left(a_{k}+1\right)=60. The common divisor of 64 and 60 is 1, 2, or 4. Case 1: (a3+1)(a4+1)(ak+1)=4\left(a_{3}+1\right)\left(a_{4}+1\right)\cdots\left(a_{k}+1\right)=4. n=2253p3p4n=2^{2}5^{3}p_{3}p_{4} or n=2253p33n=2^{2}5^{3}p_{3}^{3}. Case 2: (a3+1)(a4+1)(ak+1)=2\left(a_{3}+1\right)\left(a_{4}+1\right)\cdots\left(a_{k}+1\right)=2. No values of rr and ss work. Case 3: (a3+1)(a4+1)(ak+1)=1\left(a_{3}+1\right)\left(a_{4}+1\right)\cdots\left(a_{k}+1\right)=1. No values of rr and ss work. Therefore, n=500p3p4n=500p_{3}p_{4} or n=500p33n=500p_{3}^{3}. There are 4 possible values of nn.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.