title: Problem 512
date: 2015/04/18 22:00:00
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# [Problem 512](https://projecteuler.net/problem=512)
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[Xem đề gốc (tiếng Anh)](https://projecteuler.net/problem=512)

## **Tổng totient lũy thừa**

$f(n) = \sum_{k=1}^{n} \varphi(k^5)$. Tính $f(10^{12}) \pmod{10^9}$.

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