let X be non empty set ; :: thesis: for S being SigmaField of X
for M being sigma_Measure of S
for f being PartFunc of X,ExtREAL
for A, B being Element of S st dom f = A holds
( f is_measurable_on B iff f is_measurable_on A /\ B )
let S be SigmaField of X; :: thesis: for M being sigma_Measure of S
for f being PartFunc of X,ExtREAL
for A, B being Element of S st dom f = A holds
( f is_measurable_on B iff f is_measurable_on A /\ B )
let M be sigma_Measure of S; :: thesis: for f being PartFunc of X,ExtREAL
for A, B being Element of S st dom f = A holds
( f is_measurable_on B iff f is_measurable_on A /\ B )
let f be PartFunc of X,ExtREAL ; :: thesis: for A, B being Element of S st dom f = A holds
( f is_measurable_on B iff f is_measurable_on A /\ B )
let A, B be Element of S; :: thesis: ( dom f = A implies ( f is_measurable_on B iff f is_measurable_on A /\ B ) )
assume A1: dom f = A ; :: thesis: ( f is_measurable_on B iff f is_measurable_on A /\ B )

A2: now

let r be real number ; :: thesis: A /\ (less_dom f,(R_EAL r)) = less_dom f,(R_EAL r)

now

let x be set ; :: thesis: ( x in A /\ (less_dom f,(R_EAL r)) iff x in less_dom f,(R_EAL r) )
( x in A /\ (less_dom f,(R_EAL r)) iff ( x in A & x in less_dom f,(R_EAL r) ) ) by XBOOLE_0:def 4;
hence ( x in A /\ (less_dom f,(R_EAL r)) iff x in less_dom f,(R_EAL r) ) by A1, MESFUNC1:def 12; :: thesis: verum

end;

then ( A /\ (less_dom f,(R_EAL r)) c= less_dom f,(R_EAL r) & less_dom f,(R_EAL r) c= A /\ (less_dom f,(R_EAL r)) ) by TARSKI:def 3;
hence A /\ (less_dom f,(R_EAL r)) = less_dom f,(R_EAL r) by XBOOLE_0:def 10; :: thesis: verum

end;

hereby :: thesis: ( f is_measurable_on A /\ B implies f is_measurable_on B )

assume A3: f is_measurable_on B ; :: thesis: f is_measurable_on A /\ B

now

let r be real number ; :: thesis: (A /\ B) /\ (less_dom f,(R_EAL r)) in S
(A /\ B) /\ (less_dom f,(R_EAL r)) = B /\ (A /\ (less_dom f,(R_EAL r))) by XBOOLE_1:16
.= B /\ (less_dom f,(R_EAL r)) by A2 ;
hence (A /\ B) /\ (less_dom f,(R_EAL r)) in S by A3, MESFUNC1:def 17; :: thesis: verum

end;

hence f is_measurable_on A /\ B by MESFUNC1:def 17; :: thesis: verum

end;

assume A4: f is_measurable_on A /\ B ; :: thesis: f is_measurable_on B

now

let r be real number ; :: thesis: B /\ (less_dom f,(R_EAL r)) in S
(A /\ B) /\ (less_dom f,(R_EAL r)) = B /\ (A /\ (less_dom f,(R_EAL r))) by XBOOLE_1:16
.= B /\ (less_dom f,(R_EAL r)) by A2 ;
hence B /\ (less_dom f,(R_EAL r)) in S by A4, MESFUNC1:def 17; :: thesis: verum

end;

hence f is_measurable_on B by MESFUNC1:def 17; :: thesis: verum