Erdös, Paul; Katai, I. , On the sum sum d4(n) ,
Acta Sci. Math. 30, 313-324 (1969).

I. Kátai, P. Liardet, Additive functions with respect to expansions over the set of Gaussian integers,
Acta Arith. 99 (2001), 173-182

Umberto Cerruti, Mario Giacobini, Pierre Liardet, Prediction of Binary Sequences by Evolving Finite State Machines,
Artificial Evolution 2001: Lecture Notes in Computer Science vol. 2310 (Springer 2002), 42-53