Hueckel's edge detector finds the best-fitting ideal step edge to a given picture neighborhood by expanding the neighborhood and step edge in terms of a set of nine basis functions. A very simple case of this approach uses a 2 × 2 neighborhood and three basis functions. This case is solved explicitly using elementary methods. The magnitude of the best-fitting step edge for the neighborhood AB CD turns out to be the Roberts operator max (|A - D|, |B - C|).