def is_triangular(number: int) -> bool:
    """
    Returns True if number is a triangular number: 
    can be written as (s+1)*s/2
    https://oeis.org/A000217
    """
    t = 0
    i = 0
    while t < number:
        i += 1
        t += i
    if t == number:
        return True
    return False