$\forall$$k$, $b$:$\mathbb{N}$, $f$:($\mathbb{N}$$_{\mbox{\scriptsize $<$$k$}}$$\rightarrow\mathbb{Z}$). ($\forall$$x$:$\mathbb{N}$$_{\mbox{\scriptsize $<$$k$}}$. $b$$\leq$$f$($x$)) $\Rightarrow$ $b$$\cdot$$k$$\leq$sum($f$($x$) $\mid$ $x$ $<$ $k$)