loj125 除数函数求和 2
$原式=2\sum_{i=1}^n(i^2*{\lfloor}{\frac{n}{i}}{\rfloor})+3\sum_{i=1}^n(i*{\lfloor}{\frac{n}{i}}{\rfloor})+5\sum_{i=1}^n({\lfloor}{\frac{n}{i}}{\rfloor})$
1 #include<cstdio> 2 #include<algorithm> 3 #include<cstring> 4 #include<vector> 5 using namespace std; 6 #define fi first 7 #define se second 8 #define mp make_pair 9 #define pb push_back 10 typedef long long ll; 11 typedef unsigned long long ull; 12 typedef pair<int,int> pii; 13 #define md 998244353 14 ll n,a1,a2,a3; 15 ll calc(ll x) {return x*(x+1)%md*(2*x+1)%md*166374059%md;} 16 int main() 17 { 18 ll i,j,t; 19 scanf("%lld",&n); 20 for(i=1;i<=n;i=j+1) 21 { 22 t=n/i; 23 j=min(n,n/t); 24 a1=(a1+(calc(j)-calc(i-1)+md)%md*t%md)%md; 25 a2=(a2+(i+j)*(j-i+1)%md*499122177%md*t%md)%md; 26 a3=(a3+(j-i+1)*t)%md; 27 } 28 printf("%lld",(2*a1+3*a2+5*a3)%md); 29 return 0; 30 }
