title: Problem 443
date: 2013/11/02 16:00:00
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# [Problem 443](https://projecteuler.net/problem=443)
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[Xem đề gốc (tiếng Anh)](https://projecteuler.net/problem=443)

## **Dãy GCD**

$a_1=1$, $a_n = a_{n-1} + \gcd(n, a_{n-1})$. Tính $a(10^{15})$.

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