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Direct sums of balanced functions, perfect nonlinear functions, and orthogonal cocycles. (English) Zbl 1136.94006
Summary: Determining if a direct sum of functions inherits nonlinearity properties from its direct summands is a subtle problem. Here, we correct a statement by K. Nyberg [Lect. Notes Comput. Sci. 547, 378–386 (1991; Zbl 0766.94012)] on inheritance of balance and we use a connection between balanced derivatives and orthogonal cocycles to generalize Nyberg’s result to orthogonal cocycles. We obtain a new search criterion for PN functions and orthogonal cocycles mapping to non-cyclic abelian groups and use it to find all the orthogonal cocycles over $$\mathbb{Z}_{2}^{t}$$, $$2 \leq t \leq 4$$. We conjecture that any orthogonal cocycle over $$\mathbb{Z}_{2}^{t}, t \geq 2$$, must be multiplicative.

##### MSC:
 94A60 Cryptography 94A55 Shift register sequences and sequences over finite alphabets in information and communication theory 20J06 Cohomology of groups
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