What cases must necessarily be present in a recursive function?

Necessary components of a recursive function:

1. Base Case: The condition under which recursion stops. Without it, the function will call itself infinitely, leading to stack overflow.
2. Recursive Step: Calling the function itself with modified arguments that gradually approach the base case.

Example:

def factorial(n):
    # Base case
    if n == 0:
        return 1
    # Recursive step
    else:
        return n * factorial(n-1)