let X be non empty set ; :: thesis:
let S be SigmaField of X; :: thesis:
let M be sigma_Measure of S; :: thesis:
let A be Element of S; :: thesis:
let f be PartFunc of X,ExtREAL; :: thesis:
assume A1: ( A = dom f & f is A -measurable & f is nonnegative ) ; :: thesis:

A2: now :: thesis:

assume f is_integrable_on M ; :: thesis:
then ( -infty < Integral (M,f) & Integral (M,f) < +infty ) by MESFUNC5:96;
hence Integral (M,f) in REAL by XXREAL_0:14; :: thesis:

end;

assume A3: Integral (M,f) in REAL ; :: thesis:
A4: ( dom (max- f) = A & max- f is A -measurable ) by A1, MESFUNC2:26, MESFUNC2:def 3;
A5: ( dom (max+ f) = A & max+ f is A -measurable ) by A1, MESFUNC2:25, MESFUNC2:def 2;
for x being Element of X holds 0 <= (max+ f) . x by MESFUNC2:12;
then max+ f is nonnegative by SUPINF_2:39;
then A6: Integral (M,(max+ f)) = integral+ (M,(max+ f)) by A5, MESFUNC5:88;
A7: for x being Element of X st x in dom f holds
(max+ f) . x = f . x

let x be Element of X; :: thesis:
A8: f . x >= 0 by A1, SUPINF_2:39;
assume x in dom f ; :: thesis:
then (max+ f) . x = max ((f . x),0) by A1, A5, MESFUNC2:def 2;
hence (max+ f) . x = f . x by A8, XXREAL_0:def 10; :: thesis:

end;

then max+ f = f by A1, A5, PARTFUN1:5;
then A9: Integral (M,(max+ f)) < +infty by A3, XXREAL_0:9;
for x being Element of X holds 0 <= (max- f) . x by MESFUNC2:13;
then max- f is nonnegative by SUPINF_2:39;
then A10: Integral (M,(max- f)) = integral+ (M,(max- f)) by A4, MESFUNC5:88;
for x being Element of X st x in dom (max- f) holds
0 = (max- f) . x

let x be Element of X; :: thesis:
assume x in dom (max- f) ; :: thesis:
(max+ f) . x = f . x by A1, A5, A7, PARTFUN1:5;
hence 0 = (max- f) . x by MESFUNC2:19; :: thesis:

end;

then Integral (M,(max- f)) = 0 by A4, LPSPACE1:22;
hence f is_integrable_on M by A1, A6, A9, A10; :: thesis:

end;

hence ( Integral (M,f) in REAL iff f is_integrable_on M ) by A2; :: thesis: