Calculate the geometric sum of N - 1

Calculate the geometric sum of N - 1.

Note:

In mathematics, a geometric series is a series

with a constant ratio between successive terms.

Example:

harmonic series

def geometric_sum(n):
    if n < 0:
        return 0
    else:
        return 1 / (pow(2, n)) + geometric_sum(n - 1)

# test
print(geometric_sum(7))    # 1.9921875
print(geometric_sum(4))    # 1.9375

See also

https://www.w3resource.com/python-exercises/data-structures-and-algorithms/python-recursion-exercise-9.php