let T be non empty Hausdorff compact TopSpace; for N being net of T ex c being Point of T st c is_a_cluster_point_of N
let N be net of T; ex c being Point of T st c is_a_cluster_point_of N
defpred S1[ set , set ] means ex X being Subset of T ex a being Element of N st
( a = $1 & X = { (N . j) where j is Element of N : a <= j } & $2 = Cl X );
A1:
for e being Element of N ex u being Subset of T st S1[e,u]
consider F being Function of the carrier of N,(bool the carrier of T) such that
A2:
for e being Element of N holds S1[e,F . e]
from FUNCT_2:sch 3(A1);
reconsider RF = rng F as Subset-Family of T ;
A3:
dom F = the carrier of N
by FUNCT_2:def 1;
A4:
RF is centered
proof
thus
RF <> {}
by A3, RELAT_1:42;
FINSET_1:def 3 for b1 being set holds
( b1 = {} or not b1 c= RF or not b1 is finite or not meet b1 = {} )
defpred S2[
set ,
set ]
means ex
i being
Element of
N ex
h,
Ch being
Subset of
T st
(
i = $2 &
Ch = $1 &
h = { (N . j) where j is Element of N : i <= j } &
Ch = Cl h );
set J =
{ i where i is Element of N : ex h, Ch being Subset of T st
( h = { (N . j) where j is Element of N : i <= j } & Ch = Cl h ) } ;
let H be
set ;
( H = {} or not H c= RF or not H is finite or not meet H = {} )
assume that A5:
H <> {}
and A6:
H c= RF
and A7:
H is
finite
;
not meet H = {}
reconsider H1 =
H as non
empty set by A5;
set e = the
Element of
H1;
the
Element of
H1 in RF
by A6;
then consider x being
object such that A8:
x in dom F
and
the
Element of
H1 = F . x
by FUNCT_1:def 3;
reconsider x =
x as
Element of
N by A8;
consider X being
Subset of
T,
a being
Element of
N such that
a = x
and A9:
(
X = { (N . j) where j is Element of N : a <= j } &
F . x = Cl X )
by A2;
a in { i where i is Element of N : ex h, Ch being Subset of T st
( h = { (N . j) where j is Element of N : i <= j } & Ch = Cl h ) }
by A9;
then reconsider J =
{ i where i is Element of N : ex h, Ch being Subset of T st
( h = { (N . j) where j is Element of N : i <= j } & Ch = Cl h ) } as non
empty set ;
A10:
for
e being
Element of
H1 ex
u being
Element of
J st
S2[
e,
u]
consider Q being
Function of
H1,
J such that A16:
for
e being
Element of
H1 holds
S2[
e,
Q . e]
from FUNCT_2:sch 3(A10);
rng Q c= [#] N
then reconsider RQ =
rng Q as
Subset of
([#] N) ;
A19:
( not
[#] N is
empty &
[#] N is
directed )
by WAYBEL_0:def 6;
dom Q = H
by FUNCT_2:def 1;
then
rng Q is
finite
by A7, FINSET_1:8;
then consider i0 being
Element of
N such that
i0 in [#] N
and A20:
i0 is_>=_than RQ
by A19, WAYBEL_0:1;
for
Y being
set st
Y in H holds
N . i0 in Y
hence
not
meet H = {}
by A5, SETFAM_1:def 1;
verum
end;
RF is closed
then
meet RF <> {}
by A4, COMPTS_1:4;
then consider c being object such that
A29:
c in meet RF
by XBOOLE_0:def 1;
reconsider c = c as Point of T by A29;
take
c
; c is_a_cluster_point_of N
assume
not c is_a_cluster_point_of N
; contradiction
then consider O being a_neighborhood of c such that
A30:
not N is_often_in O
;
N is_eventually_in the carrier of T \ O
by A30, WAYBEL_0:10;
then consider s0 being Element of N such that
A31:
for j being Element of N st s0 <= j holds
N . j in the carrier of T \ O
;
consider Y being Subset of T, a being Element of N such that
A32:
( a = s0 & Y = { (N . j) where j is Element of N : a <= j } )
and
A33:
F . s0 = Cl Y
by A2;
Cl Y in RF
by A3, A33, FUNCT_1:def 3;
then A34:
c in Cl Y
by A29, SETFAM_1:def 1;
{} = O /\ Y
then
O misses Y
;
then A38:
Int O misses Y
by TOPS_1:16, XBOOLE_1:63;
( Int O is open & c in Int O )
by CONNSP_2:def 1;
hence
contradiction
by A34, A38, PRE_TOPC:def 7; verum