Eine Rekursive Universelle Funktion Für Die Primitiv-Rekursiven Funktionen. - Hilbert Levitz; Warren D. Nichols (Math. Log. Q., 1987)

A Natural Variant of Ackermann's Function. - Hilbert Levitz; Warren D. Nichols (Math. Log. Q., 1988)

A Macro Program for the Primitive Recursive Functions. - Hilbert Levitz; Warren D. Nichols; Robert F. Smith (Math. Log. Q., 1991)

On Series of Ordinals and Combinatorics. - James P. Jones; Hilbert Levitz; Warren D. Nichols (Math. Log. Q., 1997)

Algebraic Myhill-Nerode Theorems. - Warren D. Nichols; Robert G. Underwood (Theor. Comput. Sci., 2011)