T243086 最简分数列

https://www.luogu.com.cn/record/76645289

Luogu/T243086/T243086.cpp

@@ -0,0 +1,43 @@
+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+int n, p, primes[10000005], phi[10000005];
+bool vis[10000005];
+long long ans;
+
+void get_eulers(int n) {
+    phi[1] = 1;
+    for (int i = 2; i <= n; i++) {
+        if (!vis[i]) {
+            primes[p++] = i;
+            phi[i] = i - 1;
+        }
+        for (int j = 0; primes[j] * i <= n; j++) {
+            vis[primes[j] * i] = true;
+            if (i % primes[j] == 0) {
+                phi[primes[j] * i] = phi[i] * primes[j];
+                break;
+            }
+            phi[primes[j] * i] = phi[i] * (primes[j] - 1);
+        }
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    cin >> n;
+
+    get_eulers(n);
+
+    for (int i = 2; i <= n; i++) {
+        ans += phi[i];
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}