from math import sqrt :

def ispentagonal(a) :

if a <= 0 :

    return False

delt = 1 + 24 * a
x1 = (1 - sqrt(delt)) / 6
x2 = (1 + sqrt(delt)) / 6

if x1 > 0 and x1.is_integer():
    return True
if x2 > 0 and x2.is_integer():
    return True