Problem 58

58.c

@@ -0,0 +1,29 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <math.h>
+
+bool isPrime (int n) {
+    int i = 2;
+    while (i*i <= n) {
+        if (n % i == 0)
+            return false;
+        i++;
+    }
+    return true;
+}
+
+int main () {
+    int side = 7;
+    int diagonals = 13;
+    int primes = 8;
+    while ((double) primes / diagonals > 0.1) {
+        side += 2;
+        diagonals += 4;
+        for (int n = 0; n < 4; n++) {
+            if (isPrime(side*side - n*(side - 1)))
+                primes++;
+        }
+    }
+    printf ("side length: %d\n", side);
+}