// Return the ISqrt of the number, if the number is a perfect square,
// otherwise not.
func perfect_root(base int) (root int, perfect bool) {
root = euler.ISqrt(base)
perfect = (root*root == base)
return
}
func IsPentagonal(num int) bool {
sq := num*24 + 1
root := euler.ISqrt(sq)
return (root*root == sq) &&
((root+1)%6) == 0
}