Maths Olympiad Prep

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Number theory Difficulty 4.3 AIME Find the answer United States

Problem:
Let SS be the set of all positive integers whose prime factorizations only contain powers of the primes 22 and 20172017 (that is, 11, powers of 22, and powers of 20172017 are thus contained in SS). Compute
sS1s. \sum_{s \in S} \frac{1}{s}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
Since every ss can be written as 2i2017j2^{i} \cdot 2017^{j} for non-negative integers ii and jj, the given sum can be written as
(i=012i)(j=012017j). \left(\sum_{i=0}^{\infty} \frac{1}{2^{i}}\right)\left(\sum_{j=0}^{\infty} \frac{1}{2017^{j}}\right).
We can easily find the sum of these geometric series since they both have common ratio of magnitude less than 11, giving us
(1112)1112017=220172016=20171008. \left(\frac{1}{1-\frac{1}{2}}\right) \cdot \frac{1}{1-\frac{1}{2017}} = 2 \cdot \frac{2017}{2016} = \frac{2017}{1008}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.