Issleduetsya shodimost' raznostnyh shem, approksimiruyushchih uravnenie $-\Delta u+cu=f,\, c$ usloviyami periodichnosti. Pokazano, chto obychnaya shema ``krest'' s usrednennoi chas'yu, shoditsya v diskretnoi $W_{2^*}^m$-norme so skorost'yu $O(h^\alpha)$, esli reshenie ishodnoi zadachi prinadlezhit periodicheskomu prostranstvu Soboleva--Slobodeckogo $W_{2,^*}^{m+\alpha}$ ($m=0, \pm 1, \pm 2,\ldots$; $0<\alpha\leq 2$).