let X be non empty set ; :: thesis: for S being SigmaField of X
for f being PartFunc of X,REAL
for A being Element of S holds
( f is_measurable_on A iff for r being real number holds A /\ (less_dom (f,r)) in S )
let S be SigmaField of X; :: thesis: for f being PartFunc of X,REAL
for A being Element of S holds
( f is_measurable_on A iff for r being real number holds A /\ (less_dom (f,r)) in S )
let f be PartFunc of X,REAL; :: thesis: for A being Element of S holds
( f is_measurable_on A iff for r being real number holds A /\ (less_dom (f,r)) in S )
let A be Element of S; :: thesis: ( f is_measurable_on A iff for r being real number holds A /\ (less_dom (f,r)) in S )
A1: ( f is_measurable_on A iff R_EAL f is_measurable_on A ) by Def6;
thus ( f is_measurable_on A implies for r being real number holds A /\ (less_dom (f,r)) in S ) :: thesis: ( ( for r being real number holds A /\ (less_dom (f,r)) in S ) implies f is_measurable_on A )

proof

assume A2: f is_measurable_on A ; :: thesis: for r being real number holds A /\ (less_dom (f,r)) in S
let r be real number ; :: thesis: A /\ (less_dom (f,r)) in S
R_EAL r = r ;
hence A /\ (less_dom (f,r)) in S by A1, A2, MESFUNC1:def 17; :: thesis: verum

end;

( ( for r being real number holds A /\ (less_dom (f,(R_EAL r))) in S ) implies for r being real number holds A /\ (less_dom (f,(R_EAL r))) in S ) ;
hence ( ( for r being real number holds A /\ (less_dom (f,r)) in S ) implies f is_measurable_on A ) by A1, MESFUNC1:def 17; :: thesis: verum