Data commit

Task/Chowla-numbers/C-sharp/chowla-numbers.cs

@@ -0,0 +1,59 @@
+using System;
+
+namespace chowla_cs
+{
+    class Program
+    {
+        static int chowla(int n)
+        {
+            int sum = 0;
+            for (int i = 2, j; i * i <= n; i++)
+                if (n % i == 0) sum += i + (i == (j = n / i) ? 0 : j);
+            return sum;
+        }
+
+        static bool[] sieve(int limit)
+        {
+            // True denotes composite, false denotes prime.
+            // Only interested in odd numbers >= 3
+            bool[] c = new bool[limit];
+            for (int i = 3; i * 3 < limit; i += 2)
+                if (!c[i] && (chowla(i) == 0))
+                    for (int j = 3 * i; j < limit; j += 2 * i)
+                        c[j] = true;
+            return c;
+        }
+
+        static void Main(string[] args)
+        {
+            for (int i = 1; i <= 37; i++)
+                Console.WriteLine("chowla({0}) = {1}", i, chowla(i));
+            int count = 1, limit = (int)(1e7), power = 100;
+            bool[] c = sieve(limit);
+            for (int i = 3; i < limit; i += 2)
+            {
+                if (!c[i]) count++;
+                if (i == power - 1)
+                {
+                    Console.WriteLine("Count of primes up to {0,10:n0} = {1:n0}", power, count);
+                    power *= 10;
+                }
+            }
+
+            count = 0; limit = 35000000;
+            int k = 2, kk = 3, p;
+            for (int i = 2; ; i++)
+            {
+                if ((p = k * kk) > limit) break;
+                if (chowla(p) == p - 1)
+                {
+                    Console.WriteLine("{0,10:n0} is a number that is perfect", p);
+                    count++;
+                }
+                k = kk + 1; kk += k;
+            }
+            Console.WriteLine("There are {0} perfect numbers <= 35,000,000", count);
+            if (System.Diagnostics.Debugger.IsAttached) Console.ReadKey();
+        }
+    }
+}