'commit'

TangDou/AcWing_TiGao/T5/OuLaHanShu/220.cpp

@@ -0,0 +1,46 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+#define int long long
+#define endl "\n"
+const int N = 1e7 + 10; // 这家伙好大啊
+int s[N];
+
+// 筛法求欧拉函数值
+int primes[N], cnt;
+bool st[N];
+int phi[N];
+void euler(int n) {
+    phi[1] = 1;
+    for (int i = 2; i <= n; i++) {
+        if (!st[i]) {
+            primes[cnt++] = i;
+            phi[i] = i - 1;
+        }
+        for (int j = 0; primes[j] * i <= n; j++) {
+            st[primes[j] * i] = true;
+            if (i % primes[j] == 0) {
+                phi[i * primes[j]] = phi[i] * primes[j];
+                break;
+            }
+            phi[i * primes[j]] = phi[i] * (primes[j] - 1);
+        }
+    }
+}
+
+signed main() {
+    int n;
+    cin >> n;
+
+    // 欧拉函数筛法
+    euler(n);
+
+    // 前缀和
+    for (int i = 1; i <= n; i++) s[i] = s[i - 1] + phi[i];
+
+    int res = 0;
+    for (int i = 0; i < cnt; i++)
+        res += s[n / primes[i]] * 2 - 1; // 推导的公式
+
+    cout << res << endl;
+}