题面
求(sum_{i=1}^{n-1}sum_{j=i+1}^{n}gcd(i, j))
n<=4000000,数据组数T<=100
答案保证在64位带符号整数范围内（long long就好）
Sol
之前做了一道假的
先不管i，j是否有序，我们就求(sum_{i=1}^{n}sum_{j=1}^{n}gcd(i, j))
最后(ans=(ans - (n + 1) * n / 2) / 2)即可
推导
(ans=sum_{d=1}^{n}sum_{i=1}^{lfloorfrac{n}{d}
floor}mu(i)*lfloorfrac{n}{i*d}
floor^2)
(用k替换i*d，ans=sum_{k=1}^{n}lfloorfrac{n}{k}
floor^2sum_{d|k}mu(frac{k}{d})d)
(sum_{d|k}mu(frac{k}{d})d)是积性函数，线性筛即可
加上数论分块
# include <bits/stdc++.h>
# define RG register
# define IL inline
# define Zsydalao 666
# define Fill(a, b) memset(a, b, sizeof(a))
using namespace std;
typedef long long ll;
const int _(4e6 + 1);
IL ll Read(){
char c = '%'; ll x = 0, z = 1;
for(; c > '9' || c < '0'; c = getchar()) if(c == '-') z = -1;
for(; c >= '0' && c <= '9'; c = getchar()) x = x * 10 + c - '0';
return x * z;
}
int prime[_], num;
ll f[_];
bool isprime[_];
IL void Prepare(){
isprime[1] = 1; f[1] = 1;
for(RG int i = 2; i < _; ++i){
if(!isprime[i]) prime[++num] = i, f[i] = i - 1;
for(RG int j = 1; j <= num && i * prime[j] < _; ++j){
isprime[i * prime[j]] = 1;
if(i % prime[j]) f[i * prime[j]] = f[i] * f[prime[j]];
else{ f[i * prime[j]] = f[i] * prime[j]; break; }
}
}
for(RG int i = 2; i < _; ++i) f[i] += f[i - 1];
}
int main(RG int argc, RG char *argv[]){
Prepare();
while(Zsydalao == 666){
RG ll n = Read(), ans = 0;
if(!n) break;
for(RG ll k = 1, j; k <= n; k = j + 1){
j = n / (n / k);
ans += (n / k) * (n / k) * (f[j] - f[k - 1]);
}
printf("%lld
", (ans - n * (n + 1) / 2) / 2);
}
return 0;
}