let s, s9 be convergent Complex_Sequence; lim (s (#) s9) = (lim s) * (lim s9)
consider R being Real such that
A1:
0 < R
and
A2:
for n being Nat holds |.(s . n).| < R
by Th8;
A3:
0 + 0 < |.(lim s9).| + R
by A1, COMPLEX1:46, XREAL_1:8;
A4:
0 <= |.(lim s9).|
by COMPLEX1:46;
now for p being Real st 0 < p holds
ex n being set st
for m being Nat st n <= m holds
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < plet p be
Real;
( 0 < p implies ex n being set st
for m being Nat st n <= m holds
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < p )assume
0 < p
;
ex n being set st
for m being Nat st n <= m holds
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < pthen A5:
0 < p / (|.(lim s9).| + R)
by A3;
then consider n1 being
Nat such that A6:
for
m being
Nat st
n1 <= m holds
|.((s . m) - (lim s)).| < p / (|.(lim s9).| + R)
by Def6;
consider n2 being
Nat such that A7:
for
m being
Nat st
n2 <= m holds
|.((s9 . m) - (lim s9)).| < p / (|.(lim s9).| + R)
by A5, Def6;
take n =
n1 + n2;
for m being Nat st n <= m holds
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < plet m be
Nat;
( n <= m implies |.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < p )assume A8:
n <= m
;
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < p
n1 <= n1 + n2
by NAT_1:12;
then
n1 <= m
by A8, XXREAL_0:2;
then A9:
|.((s . m) - (lim s)).| <= p / (|.(lim s9).| + R)
by A6;
|.(((s . m) - (lim s)) * (lim s9)).| = |.(lim s9).| * |.((s . m) - (lim s)).|
by COMPLEX1:65;
then A10:
|.(((s . m) - (lim s)) * (lim s9)).| <= |.(lim s9).| * (p / (|.(lim s9).| + R))
by A4, A9, XREAL_1:64;
A11:
(
0 <= |.(s . m).| &
0 <= |.((s9 . m) - (lim s9)).| )
by COMPLEX1:46;
n2 <= n
by NAT_1:12;
then
n2 <= m
by A8, XXREAL_0:2;
then A12:
|.((s9 . m) - (lim s9)).| < p / (|.(lim s9).| + R)
by A7;
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| =
|.(((((s . m) * (s9 . m)) - ((s . m) * (lim s9))) + ((s . m) * (lim s9))) - ((lim s) * (lim s9))).|
by VALUED_1:5
.=
|.(((s . m) * ((s9 . m) - (lim s9))) + (((s . m) - (lim s)) * (lim s9))).|
;
then A13:
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| <= |.((s . m) * ((s9 . m) - (lim s9))).| + |.(((s . m) - (lim s)) * (lim s9)).|
by COMPLEX1:56;
|.(s . m).| < R
by A2;
then
|.(s . m).| * |.((s9 . m) - (lim s9)).| < R * (p / (|.(lim s9).| + R))
by A11, A12, XREAL_1:96;
then A14:
|.((s . m) * ((s9 . m) - (lim s9))).| < R * (p / (|.(lim s9).| + R))
by COMPLEX1:65;
(R * (p / (|.(lim s9).| + R))) + (|.(lim s9).| * (p / (|.(lim s9).| + R))) =
(p / (|.(lim s9).| + R)) * (|.(lim s9).| + R)
.=
p
by A3, XCMPLX_1:87
;
then
|.((s . m) * ((s9 . m) - (lim s9))).| + |.(((s . m) - (lim s)) * (lim s9)).| < p
by A14, A10, XREAL_1:8;
hence
|.(((s (#) s9) . m) - ((lim s) * (lim s9))).| < p
by A13, XXREAL_0:2;
verum end;
hence
lim (s (#) s9) = (lim s) * (lim s9)
by Def6; verum