let X be non empty TopSpace; :: thesis: for x, y being Point of X holds
( Cl {x} c= Cl {y} iff MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) } )

let x, y be Point of X; :: thesis: ( Cl {x} c= Cl {y} iff MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) } )
set FY = { G where G is Subset of X : ( G is open & y in G ) } ;
{ G where G is Subset of X : ( G is open & y in G ) } c= bool the carrier of X
proof
let C be object ; :: according to TARSKI:def 3 :: thesis: ( not C in { G where G is Subset of X : ( G is open & y in G ) } or C in bool the carrier of X )
assume C in { G where G is Subset of X : ( G is open & y in G ) } ; :: thesis: C in bool the carrier of X
then ex P being Subset of X st
( C = P & P is open & y in P ) ;
hence C in bool the carrier of X ; :: thesis: verum
end;
then reconsider FY = { G where G is Subset of X : ( G is open & y in G ) } as Subset-Family of X ;
set FX = { G where G is Subset of X : ( G is open & x in G ) } ;
{ G where G is Subset of X : ( G is open & x in G ) } c= bool the carrier of X
proof
let C be object ; :: according to TARSKI:def 3 :: thesis: ( not C in { G where G is Subset of X : ( G is open & x in G ) } or C in bool the carrier of X )
assume C in { G where G is Subset of X : ( G is open & x in G ) } ; :: thesis: C in bool the carrier of X
then ex P being Subset of X st
( C = P & P is open & x in P ) ;
hence C in bool the carrier of X ; :: thesis: verum
end;
then reconsider FX = { G where G is Subset of X : ( G is open & x in G ) } as Subset-Family of X ;
thus ( Cl {x} c= Cl {y} implies MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) } ) :: thesis: ( MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) } implies Cl {x} c= Cl {y} )
proof
assume Cl {x} c= Cl {y} ; :: thesis: MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) }
then A1: meet FY c= meet FX by Th51;
(Cl {y}) /\ (meet FY) c= meet FY by XBOOLE_1:17;
hence MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) } by A1; :: thesis: verum
end;
{y} c= MaxADSet y by Th12;
then A2: y in MaxADSet y by ZFMISC_1:31;
assume MaxADSet y c= meet { G where G is Subset of X : ( G is open & x in G ) } ; :: thesis:
then A3: y in meet FX by A2;
set G = (Cl {y}) ` ;
assume A4: not Cl {x} c= Cl {y} ; :: thesis: contradiction
not x in Cl {y} by ;
then x in (Cl {y}) ` by SUBSET_1:29;
then (Cl {y}) ` in FX ;
then A5: meet FX c= (Cl {y}) ` by SETFAM_1:3;
{y} c= Cl {y} by PRE_TOPC:18;
then y in Cl {y} by ZFMISC_1:31;
hence contradiction by A5, A3, XBOOLE_0:def 5; :: thesis: verum