  For any polynomial $f$ of ${\mathbb F}\_{2^n}[x]$ we introduce the following
characteristic of the distribution of its second order derivative,which extends
the differential uniformity notion:$$\delta^2(f):=\max\_{\substack{\alpha \in
{\mathbb F}\_{2^n}^{\ast} ,\alpha' \in {\mathbb F}\_{2^n}^{\ast} ,\beta \in
{\mathbb F}\_{2^n} \alpha\not=\alpha'}} \sharp\{x\in{\mathbb F}\_{2^n} \mid
D\_{\alpha,\alpha'}^2f(x)=\beta\}$$where
$D\_{\alpha,\alpha'}^2f(x):=D\_{\alpha'}(D\_{\alpha}f(x))=f(x)+f(x+\alpha)+f(x+\alpha')+f(x+\alpha+\alpha')$
is the second order derivative.Our purpose is to prove a density theorem
relative to this quantity,which is an analogue of a density theorem proved by
Voloch for the differential uniformity.
