<?php
for ($x = 2; $x <= 21; $x++) {
    $n = floor((sqrt(1 + 8 * ($x - 1)) - 1) / 2);
    $lower = $x - $n;
    $upper = $x - 1;
    printf("%d: %d - %d\n", $x, $lower, $upper);
}	
?>