let X, Y be non empty TopSpace; :: thesis: for f being Function of X,Y
for X1, X2 being non empty SubSpace of X st X = X1 union X2 holds
for x being Point of X
for x1 being Point of X1
for x2 being Point of X2 st x = x1 & x = x2 holds
( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) )
let f be Function of X,Y; :: thesis: for X1, X2 being non empty SubSpace of X st X = X1 union X2 holds
for x being Point of X
for x1 being Point of X1
for x2 being Point of X2 st x = x1 & x = x2 holds
( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) )
let X1, X2 be non empty SubSpace of X; :: thesis: ( X = X1 union X2 implies for x being Point of X
for x1 being Point of X1
for x2 being Point of X2 st x = x1 & x = x2 holds
( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) ) )
assume A1: X = X1 union X2 ; :: thesis: for x being Point of X
for x1 being Point of X1
for x2 being Point of X2 st x = x1 & x = x2 holds
( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) )
let x be Point of X; :: thesis: for x1 being Point of X1
for x2 being Point of X2 st x = x1 & x = x2 holds
( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) )
let x1 be Point of X1; :: thesis: for x2 being Point of X2 st x = x1 & x = x2 holds
( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) )
let x2 be Point of X2; :: thesis: ( x = x1 & x = x2 implies ( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) ) )
assume A2: ( x = x1 & x = x2 ) ; :: thesis: ( f is_continuous_at x iff ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) )
hence ( f is_continuous_at x implies ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) ) by Th64; :: thesis: ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 implies f is_continuous_at x )
thus ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 implies f is_continuous_at x ) :: thesis: verum

proof

assume A3: ( f | X1 is_continuous_at x1 & f | X2 is_continuous_at x2 ) ; :: thesis: f is_continuous_at x
for G being a_neighborhood of f . x ex H being a_neighborhood of x st f .: H c= G

proof

let G be a_neighborhood of f . x; :: thesis: ex H being a_neighborhood of x st f .: H c= G
A4: ( f . x = (f | X1) . x1 & f . x = (f | X2) . x2 ) by A2, FUNCT_1:72;
then consider H1 being a_neighborhood of x1 such that
A5: (f | X1) .: H1 c= G by A3, Def2;
consider H2 being a_neighborhood of x2 such that
A6: (f | X2) .: H2 c= G by A3, A4, Def2;
consider H being a_neighborhood of x such that
A7: H c= H1 \/ H2 by A1, A2, Th21;
take H ; :: thesis: f .: H c= G
the carrier of X1 c= the carrier of X by BORSUK_1:35;
then reconsider S1 = H1 as Subset of X by XBOOLE_1:1;
the carrier of X2 c= the carrier of X by BORSUK_1:35;
then reconsider S2 = H2 as Subset of X by XBOOLE_1:1;
( f .: S1 c= G & f .: S2 c= G ) by A5, A6, FUNCT_2:174;
then ( S1 c= f " G & S2 c= f " G ) by FUNCT_2:172;
then S1 \/ S2 c= f " G by XBOOLE_1:8;
then H c= f " G by A7, XBOOLE_1:1;
hence f .: H c= G by FUNCT_2:172; :: thesis: verum

end;

hence f is_continuous_at x by Def2; :: thesis: verum

end;