let x0 be real number ; :: thesis: for f being PartFunc of REAL ,REAL st f is_continuous_in x0 & f . x0 <> 0 holds
f ^ is_continuous_in x0
let f be PartFunc of REAL ,REAL ; :: thesis: ( f is_continuous_in x0 & f . x0 <> 0 implies f ^ is_continuous_in x0 )
assume that
A1: f is_continuous_in x0 and
A2: f . x0 <> 0 ; :: thesis: f ^ is_continuous_in x0
not f . x0 in {0 } by A2, TARSKI:def 1;
then A3: not x0 in f " {0 } by FUNCT_1:def 13;
let s1 be Real_Sequence; :: according to FCONT_1:def 1 :: thesis: ( rng s1 c= dom (f ^ ) & s1 is convergent & lim s1 = x0 implies ( (f ^ ) /* s1 is convergent & (f ^ ) . x0 = lim ((f ^ ) /* s1) ) )
assume that
A4: rng s1 c= dom (f ^ ) and
A5: ( s1 is convergent & lim s1 = x0 ) ; :: thesis: ( (f ^ ) /* s1 is convergent & (f ^ ) . x0 = lim ((f ^ ) /* s1) )
( (dom f) \ (f " {0 }) c= dom f & rng s1 c= (dom f) \ (f " {0 }) ) by A4, RFUNCT_1:def 8, XBOOLE_1:36;
then rng s1 c= dom f by XBOOLE_1:1;
then A6: ( f /* s1 is convergent & f . x0 = lim (f /* s1) ) by A1, A5, Def1;
then (f /* s1) " is convergent by A2, A4, RFUNCT_2:26, SEQ_2:35;
hence (f ^ ) /* s1 is convergent by A4, RFUNCT_2:27; :: thesis: (f ^ ) . x0 = lim ((f ^ ) /* s1)
x0 in dom f by A2, FUNCT_1:def 4;
then x0 in (dom f) \ (f " {0 }) by A3, XBOOLE_0:def 5;
then x0 in dom (f ^ ) by RFUNCT_1:def 8;
hence (f ^ ) . x0 = (f . x0) " by RFUNCT_1:def 8
.= lim ((f /* s1) " ) by A2, A4, A6, RFUNCT_2:26, SEQ_2:36
.= lim ((f ^ ) /* s1) by A4, RFUNCT_2:27 ;
:: thesis: verum