Number theoryDifficulty 4.3AIMEFind the answerUnited States
Problem: Let S be the set of all positive integers whose prime factorizations only contain powers of the primes 2 and 2017 (that is, 1, powers of 2, and powers of 2017 are thus contained in S). Compute s∈S∑s1.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Solution: Since every s can be written as 2i⋅2017j for non-negative integers i and j, the given sum can be written as (i=0∑∞2i1)(j=0∑∞2017j1). We can easily find the sum of these geometric series since they both have common ratio of magnitude less than 1, giving us (1−211)⋅1−201711=2⋅20162017=10082017.
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Source: MathNet,
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