let A be non empty closed_interval Subset of REAL; :: thesis: for D being Division of A holds delta D >= 0
let D be Division of A; :: thesis: delta D >= 0
consider y being Element of REAL such that
A1: y in rng D by SUBSET_1:4;
consider n being Element of NAT such that
A2: n in dom D and
y = D . n by A1, PARTFUN1:3;
n in Seg (len D) by A2, FINSEQ_1:def 3;
then n in Seg (len (upper_volume ((chi (A,A)),D))) by INTEGRA1:def 6;
then n in dom (upper_volume ((chi (A,A)),D)) by FINSEQ_1:def 3;
then (upper_volume ((chi (A,A)),D)) . n in rng (upper_volume ((chi (A,A)),D)) by FUNCT_1:def 3;
then A3: (upper_volume ((chi (A,A)),D)) . n <= max (rng (upper_volume ((chi (A,A)),D))) by XXREAL_2:def 8;
vol (divset (D,n)) = (upper_volume ((chi (A,A)),D)) . n by A2, INTEGRA1:20;
then (upper_volume ((chi (A,A)),D)) . n >= 0 by INTEGRA1:9;
hence delta D >= 0 by A3; :: thesis: verum