use strict;

my @P = ();
my %reverseP = ();

my $i = 0;
my $j = 0;
my $min = 0;

print "Starting generation...";
for ($i = 1; $i < 100000; $i++)
{
  my $Pn = pentagonal($i);
  push @P, $Pn;
  $reverseP{$Pn} = $i;
  $min = $Pn;
}
print " Done\n";

for ($i = 1; $i < 10000; $i++)
{
  for ($j = 1; $j < 10000; $j++)
  {
    if ($reverseP{$P[$i] + $P[$j]} > 0)
    {
      my $D = abs($P[$i] - $P[$j]);
      if ($reverseP{$D} > 0 && $min > $D)
      {
        print "$i : $j :: " . $P[$i] . " : " . $P[$j] . " :: " . $D . "\n";
        $min = $D;
      }
    }
  }
}

sub pentagonal
{
  my $n = shift;
  return ($n * (3 * $n - 1) / 2);
}