定义积性函数 f(x)f(x)f(x)
f(pk)=p⌊k2⌋f(p^k)=p^{\lfloor\frac{k}{2}\rfloor}f(pk)=p⌊2k⌋
求 ∑i=1nf(i)\sum_{i=1}^nf(i)∑i=1nf(i)
1≤n≤10141\le n\le10^{14}1≤n≤1014