This function is named after the German mathematician Wilhelm Ackermann and the Hungarian mathematician Rósza Péter. It has two variables, denoted m and n. Its definition looks simple, as does the algorithm used to compute its values. Each instruction seems reasonable, yet this sequence of steps quickly becomes unmanageable, producing results that are gigantic or altogether out of reach.

Wilhelm Ackermann (1896–1962).

Rósza Péter (1905–1977).