A variant of the Ford-Johnson algorithm that is more space efficient

A variant of the Ford-Johnson or merge insertion sorting algorithm that we called four Ford-Johnson (₄FJ, for short) is presented and proved to execute exactly the same number of comparisons than the Ford-Johnson algorithm. The main advantage of our algorithm is that, instead of recursively working over lists of size the half of the input, as the Ford-Johnson algorithm does, ₄FJ recursively works over lists of size the quarter of the input. This allows for implementations of data structures for coordinating the recursive calls of size only 33% of the ones needed for the Ford-Johnson algorithm.