Data commit

Task/Totient-function/C-sharp/totient-function.cs

@@ -0,0 +1,38 @@
+using static System.Console;
+using static System.Linq.Enumerable;
+
+public class Program
+{
+    static void Main()
+    {
+        for (int i = 1; i <= 25; i++) {
+            int t = Totient(i);
+            WriteLine(i + "\t" + t + (t == i - 1 ? "\tprime" : ""));
+        }
+        WriteLine();
+        for (int i = 100; i <= 100_000; i *= 10) {
+            WriteLine($"{Range(1, i).Count(x => Totient(x) + 1 == x):n0} primes below {i:n0}");
+        }
+    }
+
+    static int Totient(int n) {
+        if (n < 3) return 1;
+        if (n == 3) return 2;
+
+        int totient = n;
+
+        if ((n & 1) == 0) {
+            totient >>= 1;
+            while (((n >>= 1) & 1) == 0) ;
+        }
+
+        for (int i = 3; i * i <= n; i += 2) {
+            if (n % i == 0) {
+                totient -= totient / i;
+                while ((n /= i) % i == 0) ;
+            }
+        }
+        if (n > 1) totient -= totient / n;
+        return totient;
+    }
+}