Head recursion is a type of recursion in which the recursive call is the first operation performed within a function. In head recursion, the recursive call occurs before any other calculations or operations within the function. This results in the recursive calls being executed first and the final result being calculated as the recursion unwinds.

Head recursion has the following characteristics:

1. Recursive call as the first operation: In head-recursive functions, the recursive call is the initial operation performed in the function before any other calculations or operations. The recursive call is made at the beginning of the function before any other statements are executed.
2. Results calculated during recursion unwinding: In head recursion, the recursive calls are made first, and the results are calculated as the recursion unwinds. As the recursion progresses and reaches the base case, the results of each recursive call are combined or processed to obtain the final result.
3. Stack space grows with each recursive call: Unlike tail recursion, head recursion does not optimize the stack space usage. Each recursive call creates a new stack frame, which is added to the call stack. Consequently, head-recursive functions may consume more memory for large input sizes or deep recursion.

Example of a head-recursive function to calculate the sum of numbers from 1 to n (in Python):

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def sum_recursive(n):    if n == 0:        return 0    return n + sum_recursive(n - 1)  # Head recursive call print(sum_recursive(5))  # Output: 15

In this example, the sum_recursive function uses head recursion to calculate the sum of numbers from 1 to n. The recursive call to sum_recursive is made at the beginning of the function, and the results are accumulated as the recursion unwinds.

Head recursion is a common form of recursion and can be useful in scenarios where the results need to be processed or accumulated during the recursive calls. However, it's important to be aware of the potential memory usage when dealing with deep or large-scale recursive computations.

It's worth noting that not all recursive functions can be transformed into tail or head recursive forms. The choice between tail recursion and head recursion depends on the specific requirements of the problem and the nature of the calculations involved.