How many positive integers n≤20000 have the properties that 2n has 64 positive divisors including 1 and 2n, and 5n has 60 positive divisors including 1 and 5n?
A number or a short expression. Spacing and $ signs are ignored.
Solution
Suppose n=2r5sp3a3p4a4⋯pkak. Since 2n has 64 divisors and 5n has 60 divisors, (r+2)(s+1)(a3+1)(a4+1)⋯(ak+1)=64 and (r+1)(s+2)(a3+1)(a4+1)⋯(ak+1)=60. The common divisor of 64 and 60 is 1, 2, or 4. Case 1: (a3+1)(a4+1)⋯(ak+1)=4. n=2253p3p4 or n=2253p33. Case 2: (a3+1)(a4+1)⋯(ak+1)=2. No values of r and s work. Case 3: (a3+1)(a4+1)⋯(ak+1)=1. No values of r and s work. Therefore, n=500p3p4 or n=500p33. There are 4 possible values of n.
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