Data commit

Task/Chowla-numbers/D/chowla-numbers.d

@@ -0,0 +1,63 @@
+import std.stdio;
+
+int chowla(int n) {
+    int sum;
+    for (int i = 2, j; i * i <= n; ++i) {
+        if (n % i == 0) {
+            sum += i + (i == (j = n / i) ? 0 : j);
+        }
+    }
+    return sum;
+}
+
+bool[] sieve(int limit) {
+    // True denotes composite, false denotes prime.
+    // Only interested in odd numbers >= 3
+    auto 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;
+}
+
+void main() {
+    foreach (i; 1..38) {
+        writefln("chowla(%d) = %d", i, chowla(i));
+    }
+    int count = 1;
+    int limit = cast(int)1e7;
+    int power = 100;
+    bool[] c = sieve(limit);
+    for (int i = 3; i < limit; i += 2) {
+        if (!c[i]) {
+            count++;
+        }
+        if (i == power - 1) {
+            writefln("Count of primes up to %10d = %d", power, count);
+            power *= 10;
+        }
+    }
+
+    count = 0;
+    limit = 350_000_000;
+    int k = 2;
+    int kk = 3;
+    int p;
+    for (int i = 2; ; ++i) {
+        p = k * kk;
+        if (p > limit) {
+            break;
+        }
+        if (chowla(p) == p - 1) {
+            writefln("%10d is a number that is perfect", p);
+            count++;
+        }
+        k = kk + 1;
+        kk += k;
+    }
+    writefln("There are %d perfect numbers <= 35,000,000", count);
+}