let f be non constant standard special_circular_sequence; :: thesis: for k being Element of NAT st 1 <= k & k + 1 <= len f holds
left_cell (f,k) is convex
let k be Element of NAT ; :: thesis: ( 1 <= k & k + 1 <= len f implies left_cell (f,k) is convex )
assume ( 1 <= k & k + 1 <= len f ) ; :: thesis: left_cell (f,k) is convex
then ex i, j being Element of NAT st
( i <= len (GoB f) & j <= width (GoB f) & cell ((GoB f),i,j) = left_cell (f,k) ) by GOBOARD9:14;
hence left_cell (f,k) is convex by Th21; :: thesis: verum