可以写成 $f(0)=0,f(n)=1+\sum_{k=2}^{20210926}f(\lfloor n/k\rfloor)$，整除分块递归求解，时间复杂度 $O(n^{3/4})$。
也可以考虑式子的含义，也就是若干个 $2$ 到 $20210926$ 的整数相乘不超过 $n$ 的方案数。忽略上界的话就很简单，而由于上界很大，只需要枚举容斥一下，容易做到 $O(n^{2/3+\epsilon})$。
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Discussion #158 for Problem #6740. Function
Type: Editorial
Status: Open
Posted by: jiangly
Posted at: 2025-12-12 23:42:39
Last updated: 2025-12-12 23:42:45
题解
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