# ECNU Foreigners

## Problem B

Solved by ultmaster. 02:42 (+)

## Problem C

Solved by zerol. 00:47 (+1)

$\begin{eqnarray} &&\sum_{i=1}^N \sum_{j=1}^i j^2 \mu^2(j) \\ &=& \sum_{i=1}^N (N-i+1) i^2 \mu^2(i) \\ &=& \sum_{i=1}^N (N-i+1) i^2 \sum_{d^2 | i} \mu(d) \\ &=& \sum_{d=1}^{\lfloor \sqrt N \rfloor} \mu(d) \sum_{k=1}^{\lfloor \frac{N}{d^2} \rfloor} (N-d^2k+1)d^4k^2 \end{eqnarray}$

## Problem D

Solved by ultmaster. 02:32 (+3)

## Problem E

Solved by kblack. 04:42 (+8)

## Problem F

Solved by ultmaster. 00:35 (+)

## Problem G

Solved by zerol. 01:06 (+)

## Problem I

Solved by kblack. 02:01 (+)

## Problem J

Solved by zerol. 01:36 (+)

## Problem K

Solved by kblack. 00:44 (+2)

# One,Two,Three,AK

oxx仅贡献若干模板题以及一道签到……

## Problem A

Unsolved. 3:35:49(-1)

## Problem B

Solved by oxx1108. 4:32:55(+3)

## Problem C

Solved by dreamcloud & oxx1108. 3:35:01(+3)

$\begin{eqnarray} &&h(n) = \sum_{x=1}^{n}\sum_{i=1}^{x}i^2\\ &=&\sum_{x=1}^{n}\frac{x\times(x+1)\times(2x+1) }{6}\\ \end{eqnarray}$

$\begin{eqnarray} &&s(n) = \sum_{i,i=k\times x^2, x\gt1}^{n} i^2\\ &=& \sum_{x = 2}^{x^2 \le n}x^4 \times mu(x)\sum_{i = 1}^{\lfloor \frac{n}{x^2}\rfloor}i^2 \end{eqnarray}$

## Problem E

Solved by oxx1108. 1:04:33(+1)

## Problem F

Solved by oxx1108. 2:12:31(+)

## Problem G

Solved by dreamcloud. 1:41:26(+1)

Unsolved.

## Problem J

Upsolved by dreamcloud

## Problem K

Solved by oxx1108. 0:23:18(+2)