I have made this post from the Algorithmic Toolbox course on Coursera and I am sharing the transcript of the part :

Although this is a recursive routine that breaks the problem into smaller problems, some would argue that this shouldn't be called divide and conquer. They claim that a divide and conquer algorithm should divide the problem into a smaller subproblem, where the smaller subproblem is some constant fraction of the original problem. In this case the sub-problem isn't 50%, or 80%, or even 95% of the original problem size. Instead, it's just one smaller than the original problem size. I don't know, maybe we should call this algorithm subtract and conquer rather than divide and conquer.