  Given X,Y two Q-vector spaces, and f:X -> Y, we study under which conditions
on the sets $B_k\subseteq X$, k=1,...,s, if $\Delta_{h_1h_2... h_s}f(x)=0$ for
all x in X and h_k in B_k, k=1,2,...,s, then $\Delta_{h_1h_2... h_s}f(x)=0$ for
all (x,h_1,...,h_s) in X^{s+1}.
