Bent function

A bent function is a boolean function of $n$ variables that have nonlinearity equal to $2^{n-1}-2^{n/2-1}$. Walsh-Adamar coefficients of bent function are equal to $\pm 2^{n/2}$. This gives the alternative definition of bent functions. Bent functions have even number of variables and achive the bound of maximal possible nonlinearity. This makes them a good blocks for cryptographics stream cyphers. Bent functions is a specific case of plateaued functions.