Write a Python program to calculate the geometric sum of n-1.Note: In mathematics, a geometric series is a series with a constant ratio between successive terms.

Sure, here is a simple Python program that calculates the geometric sum of n-1. This program assumes that the first term (a) is 1 and the common ratio (r) is 1/2.

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

# Test the function
print(geometric_sum(3))  # Output: 0.875

In this program, the geometric_sum function is a recursive function that calculates the geometric sum of n-1. The base case for the recursion is when n is less than 0, in which case the function returns 0. For n greater than or equal to 0, the function calculates the nth term of the geometric series (which is 1/(2**n)) and adds it to the geometric sum of the previous n-1 terms.