[euler/prob021] add problem 21

Author: gerdr

euler/prob021-gerdr.pl

@@ -0,0 +1,22 @@
+use v6;
+
+sub d(Int \n) {
+	my $sum = 1;
+	my \sqrt-n = sqrt n;
+
+	for 2..Int(sqrt-n) -> \a {
+		my \b = n div a;
+		$sum += a + b if a * b == n;
+	}
+
+	sqrt-n ~~ Int ?? $sum - sqrt-n !! $sum
+}
+
+my $sum = 0;
+
+for 1..10_000 -> \a {
+	my \b = d(a);
+	$sum += a + b if a < b and d(b) == a;
+}
+
+say $sum;