Create 1205E.cpp

Author: tmwilliamlin168

CodeForces/1205E.cpp

@@ -0,0 +1,45 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+#define ll long long
+
+const int mxN=1e5, M=1e9+7;
+int n, k;
+ll pk[mxN], mb[mxN], ans;
+bool c[mxN];
+
+ll iv(ll a, ll m) {
+	return a<2?1:(1-iv(m%a, a)*m)/a+m;
+}
+
+int main() {
+	ios::sync_with_stdio(0);
+	cin.tie(0);
+
+	cin >> n >> k;
+	pk[1]=iv(k, M);
+	for(int i=2; i<n; ++i)
+		pk[i]=pk[i-1]*pk[1]%M;
+	fill(mb, mb+n, 1);
+	for(int i=2; i<n; ++i) {
+		if(c[i])
+			continue;
+		for(int j=i; j<n; j+=i) {
+			if(j/i%i)
+				mb[j]=mb[j]*(M-1)%M;
+			else
+				mb[j]=0;
+			c[j]=1;
+		}
+	}
+	for(int i=1; i<n; ++i) {
+		for(int t=i; t<2*n-1; t+=i) {
+			for(int s=t; s<2*n-1; s+=t) {
+				int g=s/t, l=max(t-(n-1)/g-1, 0), r=min(t-1, (n-1)/g);
+				if(l<r&&g<n)
+					ans=(ans+(r/i-l/i)*pk[n-max(s-n, g)]%M*mb[i])%M;
+			}
+		}
+	}
+	cout << ans;
+}