（不是 btd，因为数列的介绍：a_1, a_2, ..., is a permutation of the positive integers such that the average of each initial segment is an integer, **using the greedy algorithm** to define a_n.）
求 OEIS 中 [A019444](https://oeis.org/A019444) 的第 $n$ 项快速求解方法，$n\le 10^{15}$
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by Spasmodic @ 2022-05-23 00:10:08