let Y be Subset of REAL; :: thesis: for f being PartFunc of REAL,REAL st f is_differentiable_on Y holds
Y is open
let f be PartFunc of REAL,REAL; :: thesis: ( f is_differentiable_on Y implies Y is open )
assume A1: f is_differentiable_on Y ; :: thesis: Y is open

now

let x0 be real number ; :: thesis: ( x0 in Y implies ex N being Neighbourhood of x0 st N c= Y )
assume x0 in Y ; :: thesis: ex N being Neighbourhood of x0 st N c= Y
then f | Y is_differentiable_in x0 by A1, Def7;
then consider N being Neighbourhood of x0 such that
A2: N c= dom (f | Y) and
ex L being LINEAR ex R being REST st
for x being Real st x in N holds
((f | Y) . x) - ((f | Y) . x0) = (L . (x - x0)) + (R . (x - x0)) by Def5;
take N = N; :: thesis: N c= Y
dom (f | Y) = (dom f) /\ Y by RELAT_1:61;
then dom (f | Y) c= Y by XBOOLE_1:17;
hence N c= Y by A2, XBOOLE_1:1; :: thesis: verum

end;

hence Y is open by RCOMP_1:20; :: thesis: verum