let f be PartFunc of REAL,REAL; :: thesis: for x0, g2 being Real st f is_Lcontinuous_in x0 & f . x0 <> g2 & ex r being Real st
( r > 0 & [.(x0 - r),x0.] c= dom f ) holds
ex r1 being Real st
( r1 > 0 & [.(x0 - r1),x0.] c= dom f & ( for g being Real st g in [.(x0 - r1),x0.] holds
f . g <> g2 ) )
let x0, g2 be Real; :: thesis: ( f is_Lcontinuous_in x0 & f . x0 <> g2 & ex r being Real st
( r > 0 & [.(x0 - r),x0.] c= dom f ) implies ex r1 being Real st
( r1 > 0 & [.(x0 - r1),x0.] c= dom f & ( for g being Real st g in [.(x0 - r1),x0.] holds
f . g <> g2 ) ) )
assume that
A1: f is_Lcontinuous_in x0 and
A2: f . x0 <> g2 ; :: thesis: ( for r being Real holds
( not r > 0 or not [.(x0 - r),x0.] c= dom f ) or ex r1 being Real st
( r1 > 0 & [.(x0 - r1),x0.] c= dom f & ( for g being Real st g in [.(x0 - r1),x0.] holds
f . g <> g2 ) ) )
given r being Real such that A3: r > 0 and
A4: [.(x0 - r),x0.] c= dom f ; :: thesis: ex r1 being Real st
( r1 > 0 & [.(x0 - r1),x0.] c= dom f & ( for g being Real st g in [.(x0 - r1),x0.] holds
f . g <> g2 ) )
defpred S₁[ Element of NAT , set ] means ( $2 in [.(x0 - (r / ($1 + 1))),x0.] & $2 in dom f & f . $2 = g2 );
assume A5: for r1 being Real st r1 > 0 & [.(x0 - r1),x0.] c= dom f holds
ex g being Real st
( g in [.(x0 - r1),x0.] & f . g = g2 ) ; :: thesis: contradiction
A6: for n being Element of NAT ex g being Real st S₁[n,g]

proof

let n be Element of NAT ; :: thesis: ex g being Real st S₁[n,g]
x0 - r <= x0 by A3, XREAL_1:44;
then A7: x0 in [.(x0 - r),x0.] by XXREAL_1:1;
A8: 0 <= n by NAT_1:2;
then 0 + 1 <= n + 1 by XREAL_1:7;
then r / (n + 1) <= r / 1 by A3, XREAL_1:118;
then A9: x0 - r <= x0 - (r / (n + 1)) by XREAL_1:13;
x0 - (r / (n + 1)) <= x0 by A3, A8, XREAL_1:44, XREAL_1:139;
then x0 - (r / (n + 1)) in { g1 where g1 is Real : ( x0 - r <= g1 & g1 <= x0 ) } by A9;
then x0 - (r / (n + 1)) in [.(x0 - r),x0.] by RCOMP_1:def 1;
then [.(x0 - (r / (n + 1))),x0.] c= [.(x0 - r),x0.] by A7, XXREAL_2:def 12;
then A10: [.(x0 - (r / (n + 1))),x0.] c= dom f by A4, XBOOLE_1:1;
then consider g being Real such that
A11: ( g in [.(x0 - (r / (n + 1))),x0.] & f . g = g2 ) by A3, A5, A8, XREAL_1:139;
take g ; :: thesis: S₁[n,g]
thus S₁[n,g] by A10, A11; :: thesis: verum

end;

consider a being Real_Sequence such that
A12: for n being Element of NAT holds S₁[n,a . n] from FUNCT_2:sch 3(A6);
A13: rng a c= (left_open_halfline x0) /\ (dom f)

proof

let x be set ; :: according to TARSKI:def 3 :: thesis: ( not x in rng a or x in (left_open_halfline x0) /\ (dom f) )
assume x in rng a ; :: thesis: x in (left_open_halfline x0) /\ (dom f)
then consider n being Element of NAT such that
A14: x = a . n by FUNCT_2:113;
a . n in [.(x0 - (r / (n + 1))),x0.] by A12;
then a . n in { g1 where g1 is Real : ( x0 - (r / (n + 1)) <= g1 & g1 <= x0 ) } by RCOMP_1:def 1;
then A15: ex g1 being Real st
( g1 = a . n & x0 - (r / (n + 1)) <= g1 & g1 <= x0 ) ;
a . n <> x0 by A2, A12;
then a . n < x0 by A15, XXREAL_0:1;
then a . n in { g1 where g1 is Real : g1 < x0 } ;
then A16: a . n in left_open_halfline x0 by XXREAL_1:229;
a . n in dom f by A12;
hence x in (left_open_halfline x0) /\ (dom f) by A14, A16, XBOOLE_0:def 4; :: thesis: verum

end;

A17: (left_open_halfline x0) /\ (dom f) c= dom f by XBOOLE_1:17;
A18: for n being Element of NAT holds (f /* a) . n = g2

proof

let n be Element of NAT ; :: thesis: (f /* a) . n = g2
thus (f /* a) . n = f . (a . n) by A13, A17, FUNCT_2:108, XBOOLE_1:1
.= g2 by A12 ; :: thesis: verum

end;

now

let n be Nat; :: thesis: (f /* a) . n = (f /* a) . (n + 1)
n in NAT by ORDINAL1:def 12;
then (f /* a) . n = g2 by A18;
hence (f /* a) . n = (f /* a) . (n + 1) by A18; :: thesis: verum

end;

then A19: lim (f /* a) = (f /* a) . 0 by SEQ_4:26, VALUED_0:25
.= g2 by A18 ;
reconsider d = NAT --> x0 as Real_Sequence ;
deffunc H₁( Element of NAT ) -> Element of REAL = x0 - (r / ($1 + 1));
consider b being Real_Sequence such that
A20: for n being Element of NAT holds b . n = H₁(n) from SEQ_1:sch 1();

A21: now

let n be Element of NAT ; :: thesis: ( b . n <= a . n & a . n <= d . n )
a . n in [.(x0 - (r / (n + 1))),x0.] by A12;
then a . n in { g1 where g1 is Real : ( x0 - (r / (n + 1)) <= g1 & g1 <= x0 ) } by RCOMP_1:def 1;
then ex g1 being Real st
( g1 = a . n & x0 - (r / (n + 1)) <= g1 & g1 <= x0 ) ;
hence ( b . n <= a . n & a . n <= d . n ) by A20, FUNCOP_1:7; :: thesis: verum

end;

A22: lim d = d . 0 by SEQ_4:26
.= x0 by FUNCOP_1:7 ;
( b is convergent & lim b = x0 ) by A20, FCONT_3:5;
then ( a is convergent & lim a = x0 ) by A22, A21, SEQ_2:19, SEQ_2:20;
hence contradiction by A1, A2, A13, A19, Def1; :: thesis: verum