:: Introduction to Lattice Theory
:: by Stanis{\l}aw \.Zukowski
::
:: Received April 14, 1989
:: Copyright (c) 1990-2021 Association of Mizar Users


definition
attr c1 is strict ;
struct /\-SemiLattStr -> 1-sorted ;
aggr /\-SemiLattStr(# carrier, L_meet #) -> /\-SemiLattStr ;
sel L_meet c1 -> BinOp of the carrier of c1;
end;

definition
attr c1 is strict ;
struct \/-SemiLattStr -> 1-sorted ;
aggr \/-SemiLattStr(# carrier, L_join #) -> \/-SemiLattStr ;
sel L_join c1 -> BinOp of the carrier of c1;
end;

definition
attr c1 is strict ;
struct LattStr -> /\-SemiLattStr , \/-SemiLattStr ;
aggr LattStr(# carrier, L_join, L_meet #) -> LattStr ;
end;

registration
let D be non empty set ;
let u be BinOp of D;
cluster \/-SemiLattStr(# D,u #) -> non empty ;
coherence
not \/-SemiLattStr(# D,u #) is empty ;
cluster /\-SemiLattStr(# D,u #) -> non empty ;
coherence
not /\-SemiLattStr(# D,u #) is empty ;
end;

registration
let D be non empty set ;
let u, n be BinOp of D;
cluster LattStr(# D,u,n #) -> non empty ;
coherence
not LattStr(# D,u,n #) is empty ;
end;

registration
cluster 1 -element strict for \/-SemiLattStr ;
existence
ex b1 being \/-SemiLattStr st
( b1 is 1 -element & b1 is strict )
proof end;
cluster 1 -element strict for /\-SemiLattStr ;
existence
ex b1 being /\-SemiLattStr st
( b1 is 1 -element & b1 is strict )
proof end;
cluster 1 -element strict for LattStr ;
existence
ex b1 being LattStr st
( b1 is 1 -element & b1 is strict )
proof end;
end;

definition
let G be non empty \/-SemiLattStr ;
let p, q be Element of G;
func p "\/" q -> Element of G equals :: LATTICES:def 1
 the L_join of G . (p,q);
coherence
the L_join of G . (p,q) is Element of G ;
end;

:: deftheorem defines "\/" LATTICES:def 1 :
for G being non empty \/-SemiLattStr
for p, q being Element of G holds p "\/" q = the L_join of G . (p,q);

definition
let G be non empty /\-SemiLattStr ;
let p, q be Element of G;
func p "/\" q -> Element of G equals :: LATTICES:def 2
 the L_meet of G . (p,q);
coherence
the L_meet of G . (p,q) is Element of G ;
end;

:: deftheorem defines "/\" LATTICES:def 2 :
for G being non empty /\-SemiLattStr
for p, q being Element of G holds p "/\" q = the L_meet of G . (p,q);

definition
let G be non empty \/-SemiLattStr ;
let p, q be Element of G;
pred p [= q means :Def3: :: LATTICES:def 3
p "\/" q = q;
end;

:: deftheorem Def3 defines [= LATTICES:def 3 :
for G being non empty \/-SemiLattStr
for p, q being Element of G holds
( p [= q iff p "\/" q = q );

Lm1: for uu, nn being BinOp of (bool {})
for x, y being Element of LattStr(# (bool {}),uu,nn #) holds x = y
proof end;

Lm2: for n being BinOp of (bool {})
for x, y being Element of \/-SemiLattStr(# (bool {}),n #) holds x = y
proof end;

Lm3: for n being BinOp of (bool {})
for x, y being Element of /\-SemiLattStr(# (bool {}),n #) holds x = y
proof end;

definition
let IT be non empty \/-SemiLattStr ;
attr IT is join-commutative means :Def4: :: LATTICES:def 4
 for a, b being Element of IT holds a "\/" b = b "\/" a;
attr IT is join-associative means :Def5: :: LATTICES:def 5
 for a, b, c being Element of IT holds a "\/" (b "\/" c) = (a "\/" b) "\/" c;
end;

:: deftheorem Def4 defines join-commutative LATTICES:def 4 :
for IT being non empty \/-SemiLattStr holds
( IT is join-commutative iff for a, b being Element of IT holds a "\/" b = b "\/" a );

:: deftheorem Def5 defines join-associative LATTICES:def 5 :
for IT being non empty \/-SemiLattStr holds
( IT is join-associative iff for a, b, c being Element of IT holds a "\/" (b "\/" c) = (a "\/" b) "\/" c );

definition
let IT be non empty /\-SemiLattStr ;
attr IT is meet-commutative means :Def6: :: LATTICES:def 6
 for a, b being Element of IT holds a "/\" b = b "/\" a;
attr IT is meet-associative means :Def7: :: LATTICES:def 7
 for a, b, c being Element of IT holds a "/\" (b "/\" c) = (a "/\" b) "/\" c;
end;

:: deftheorem Def6 defines meet-commutative LATTICES:def 6 :
for IT being non empty /\-SemiLattStr holds
( IT is meet-commutative iff for a, b being Element of IT holds a "/\" b = b "/\" a );

:: deftheorem Def7 defines meet-associative LATTICES:def 7 :
for IT being non empty /\-SemiLattStr holds
( IT is meet-associative iff for a, b, c being Element of IT holds a "/\" (b "/\" c) = (a "/\" b) "/\" c );

definition
let IT be non empty LattStr ;
attr IT is meet-absorbing means :Def8: :: LATTICES:def 8
 for a, b being Element of IT holds (a "/\" b) "\/" b = b;
attr IT is join-absorbing means :Def9: :: LATTICES:def 9
 for a, b being Element of IT holds a "/\" (a "\/" b) = a;
end;

:: deftheorem Def8 defines meet-absorbing LATTICES:def 8 :
for IT being non empty LattStr holds
( IT is meet-absorbing iff for a, b being Element of IT holds (a "/\" b) "\/" b = b );

:: deftheorem Def9 defines join-absorbing LATTICES:def 9 :
for IT being non empty LattStr holds
( IT is join-absorbing iff for a, b being Element of IT holds a "/\" (a "\/" b) = a );

definition
let IT be non empty LattStr ;
attr IT is Lattice-like means :: LATTICES:def 10
( IT is join-commutative & IT is join-associative & IT is meet-absorbing & IT is meet-commutative & IT is meet-associative & IT is join-absorbing );
end;

:: deftheorem defines Lattice-like LATTICES:def 10 :
for IT being non empty LattStr holds
( IT is Lattice-like iff ( IT is join-commutative & IT is join-associative & IT is meet-absorbing & IT is meet-commutative & IT is meet-associative & IT is join-absorbing ) );

registration
cluster non empty Lattice-like -> non empty join-commutative join-associative meet-commutative meet-associative meet-absorbing join-absorbing for LattStr ;
coherence
for b1 being non empty LattStr st b1 is Lattice-like holds
( b1 is join-commutative & b1 is join-associative & b1 is meet-absorbing & b1 is meet-commutative & b1 is meet-associative & b1 is join-absorbing ) ;
cluster non empty join-commutative join-associative meet-commutative meet-associative meet-absorbing join-absorbing -> non empty Lattice-like for LattStr ;
coherence
for b1 being non empty LattStr st b1 is join-commutative & b1 is join-associative & b1 is meet-absorbing & b1 is meet-commutative & b1 is meet-associative & b1 is join-absorbing holds
b1 is Lattice-like ;
end;

registration
cluster non empty strict join-commutative join-associative for \/-SemiLattStr ;
existence
ex b1 being non empty \/-SemiLattStr st
( b1 is strict & b1 is join-commutative & b1 is join-associative )
proof end;
cluster non empty strict meet-commutative meet-associative for /\-SemiLattStr ;
existence
ex b1 being non empty /\-SemiLattStr st
( b1 is strict & b1 is meet-commutative & b1 is meet-associative )
proof end;
cluster non empty strict Lattice-like for LattStr ;
existence
ex b1 being non empty LattStr st
( b1 is strict & b1 is Lattice-like )
proof end;
end;

definition
mode Lattice is non empty Lattice-like LattStr ;
end;

definition
let L be non empty join-commutative \/-SemiLattStr ;
let a, b be Element of L;
:: original: "\/"
redefine func a "\/" b -> Element of L;
commutativity
for a, b being Element of L holds a "\/" b = b "\/" a by Def4;
end;

definition
let L be non empty meet-commutative /\-SemiLattStr ;
let a, b be Element of L;
:: original: "/\"
redefine func a "/\" b -> Element of L;
commutativity
for a, b being Element of L holds a "/\" b = b "/\" a by Def6;
end;

definition
let IT be non empty LattStr ;
attr IT is distributive means :Def11: :: LATTICES:def 11
 for a, b, c being Element of IT holds a "/\" (b "\/" c) = (a "/\" b) "\/" (a "/\" c);
end;

:: deftheorem Def11 defines distributive LATTICES:def 11 :
for IT being non empty LattStr holds
( IT is distributive iff for a, b, c being Element of IT holds a "/\" (b "\/" c) = (a "/\" b) "\/" (a "/\" c) );

definition
let IT be non empty LattStr ;
attr IT is modular means :: LATTICES:def 12
 for a, b, c being Element of IT st a [= c holds
a "\/" (b "/\" c) = (a "\/" b) "/\" c;
end;

:: deftheorem defines modular LATTICES:def 12 :
for IT being non empty LattStr holds
( IT is modular iff for a, b, c being Element of IT st a [= c holds
a "\/" (b "/\" c) = (a "\/" b) "/\" c );

definition
let IT be non empty /\-SemiLattStr ;
attr IT is lower-bounded means :Def13: :: LATTICES:def 13
 ex c being Element of IT st
for a being Element of IT holds
( c "/\" a = c & a "/\" c = c );
end;

:: deftheorem Def13 defines lower-bounded LATTICES:def 13 :
for IT being non empty /\-SemiLattStr holds
( IT is lower-bounded iff ex c being Element of IT st
for a being Element of IT holds
( c "/\" a = c & a "/\" c = c ) );

definition
let IT be non empty \/-SemiLattStr ;
attr IT is upper-bounded means :Def14: :: LATTICES:def 14
 ex c being Element of IT st
for a being Element of IT holds
( c "\/" a = c & a "\/" c = c );
end;

:: deftheorem Def14 defines upper-bounded LATTICES:def 14 :
for IT being non empty \/-SemiLattStr holds
( IT is upper-bounded iff ex c being Element of IT st
for a being Element of IT holds
( c "\/" a = c & a "\/" c = c ) );

Lm4: for n, u being BinOp of (bool {}) holds LattStr(# (bool {}),n,u #) is Lattice
proof end;

registration
cluster non empty strict join-commutative join-associative meet-commutative meet-associative meet-absorbing join-absorbing Lattice-like distributive modular lower-bounded upper-bounded for LattStr ;
existence
ex b1 being Lattice st
( b1 is strict & b1 is distributive & b1 is lower-bounded & b1 is upper-bounded & b1 is modular )
proof end;
end;

definition
mode D_Lattice is distributive Lattice;
mode M_Lattice is modular Lattice;
mode 0_Lattice is lower-bounded Lattice;
mode 1_Lattice is upper-bounded Lattice;
end;

Lm5: for n, u being BinOp of (bool {}) holds LattStr(# (bool {}),n,u #) is 0_Lattice
proof end;

Lm6: for n, u being BinOp of (bool {}) holds LattStr(# (bool {}),n,u #) is 1_Lattice
proof end;

definition
let IT be non empty LattStr ;
attr IT is bounded means :: LATTICES:def 15
( IT is lower-bounded & IT is upper-bounded );
end;

:: deftheorem defines bounded LATTICES:def 15 :
for IT being non empty LattStr holds
( IT is bounded iff ( IT is lower-bounded & IT is upper-bounded ) );

registration
cluster non empty lower-bounded upper-bounded -> non empty bounded for LattStr ;
coherence
for b1 being non empty LattStr st b1 is lower-bounded & b1 is upper-bounded holds
b1 is bounded ;
cluster non empty bounded -> non empty lower-bounded upper-bounded for LattStr ;
coherence
for b1 being non empty LattStr st b1 is bounded holds
( b1 is lower-bounded & b1 is upper-bounded ) ;
end;

registration
cluster non empty strict join-commutative join-associative meet-commutative meet-associative meet-absorbing join-absorbing Lattice-like bounded for LattStr ;
existence
ex b1 being Lattice st
( b1 is bounded & b1 is strict )
proof end;
end;

definition
mode 01_Lattice is bounded Lattice;
end;

definition
let L be non empty /\-SemiLattStr ;
assume A1: L is lower-bounded ;
func Bottom L -> Element of L means :Def16: :: LATTICES:def 16
 for a being Element of L holds
( it "/\" a = it & a "/\" it = it );
existence
ex b1 being Element of L st
for a being Element of L holds
( b1 "/\" a = b1 & a "/\" b1 = b1 ) by A1;
uniqueness
for b1, b2 being Element of L st ( for a being Element of L holds
( b1 "/\" a = b1 & a "/\" b1 = b1 ) ) & ( for a being Element of L holds
( b2 "/\" a = b2 & a "/\" b2 = b2 ) ) holds
b1 = b2
proof end;
end;

:: deftheorem Def16 defines Bottom LATTICES:def 16 :
for L being non empty /\-SemiLattStr st L is lower-bounded holds
for b2 being Element of L holds
( b2 = Bottom L iff for a being Element of L holds
( b2 "/\" a = b2 & a "/\" b2 = b2 ) );

definition
let L be non empty \/-SemiLattStr ;
assume A1: L is upper-bounded ;
func Top L -> Element of L means :Def17: :: LATTICES:def 17
 for a being Element of L holds
( it "\/" a = it & a "\/" it = it );
existence
ex b1 being Element of L st
for a being Element of L holds
( b1 "\/" a = b1 & a "\/" b1 = b1 ) by A1;
uniqueness
for b1, b2 being Element of L st ( for a being Element of L holds
( b1 "\/" a = b1 & a "\/" b1 = b1 ) ) & ( for a being Element of L holds
( b2 "\/" a = b2 & a "\/" b2 = b2 ) ) holds
b1 = b2
proof end;
end;

:: deftheorem Def17 defines Top LATTICES:def 17 :
for L being non empty \/-SemiLattStr st L is upper-bounded holds
for b2 being Element of L holds
( b2 = Top L iff for a being Element of L holds
( b2 "\/" a = b2 & a "\/" b2 = b2 ) );

definition
let L be non empty LattStr ;
let a, b be Element of L;
pred a is_a_complement_of b means :: LATTICES:def 18
( a "\/" b = Top L & b "\/" a = Top L & a "/\" b = Bottom L & b "/\" a = Bottom L );
end;

:: deftheorem defines is_a_complement_of LATTICES:def 18 :
for L being non empty LattStr
for a, b being Element of L holds
( a is_a_complement_of b iff ( a "\/" b = Top L & b "\/" a = Top L & a "/\" b = Bottom L & b "/\" a = Bottom L ) );

definition
let IT be non empty LattStr ;
attr IT is complemented means :Def19: :: LATTICES:def 19
 for b being Element of IT ex a being Element of IT st a is_a_complement_of b;
end;

:: deftheorem Def19 defines complemented LATTICES:def 19 :
for IT being non empty LattStr holds
( IT is complemented iff for b being Element of IT ex a being Element of IT st a is_a_complement_of b );

registration
cluster non empty strict join-commutative join-associative meet-commutative meet-associative meet-absorbing join-absorbing Lattice-like bounded complemented for LattStr ;
existence
ex b1 being Lattice st
( b1 is bounded & b1 is complemented & b1 is strict )
proof end;
end;

definition
mode C_Lattice is complemented 01_Lattice;
end;

definition
let IT be non empty LattStr ;
attr IT is Boolean means :: LATTICES:def 20
( IT is bounded & IT is complemented & IT is distributive );
end;

:: deftheorem defines Boolean LATTICES:def 20 :
for IT being non empty LattStr holds
( IT is Boolean iff ( IT is bounded & IT is complemented & IT is distributive ) );

registration
cluster non empty Boolean -> non empty distributive bounded complemented for LattStr ;
coherence
for b1 being non empty LattStr st b1 is Boolean holds
( b1 is bounded & b1 is complemented & b1 is distributive ) ;
cluster non empty distributive bounded complemented -> non empty Boolean for LattStr ;
coherence
for b1 being non empty LattStr st b1 is bounded & b1 is complemented & b1 is distributive holds
b1 is Boolean ;
end;

registration
cluster non empty strict join-commutative join-associative meet-commutative meet-associative meet-absorbing join-absorbing Lattice-like Boolean for LattStr ;
existence
ex b1 being Lattice st
( b1 is Boolean & b1 is strict )
proof end;
end;

definition
mode B_Lattice is Boolean Lattice;
end;

registration
let L be non empty meet-commutative meet-absorbing join-absorbing LattStr ;
let a be Element of L;
reduce a "\/" a to a;
reducibility
a "\/" a = a
proof end;
end;

registration
let L be non empty meet-commutative meet-absorbing join-absorbing LattStr ;
let a be Element of L;
reduce a "/\" a to a;
reducibility
a "/\" a = a
proof end;
end;

theorem :: LATTICES:1
canceled;

theorem :: LATTICES:2
canceled;

::$CT 2
theorem Th1: :: LATTICES:3
for L being Lattice holds
( ( for a, b, c being Element of L holds a "/\" (b "\/" c) = (a "/\" b) "\/" (a "/\" c) ) iff for a, b, c being Element of L holds a "\/" (b "/\" c) = (a "\/" b) "/\" (a "\/" c) )
proof end;

theorem Th2: :: LATTICES:4
for L being non empty meet-absorbing join-absorbing LattStr
for a, b being Element of L holds
( a [= b iff a "/\" b = a ) by Def8, Def9;

theorem Th3: :: LATTICES:5
for L being non empty join-associative meet-commutative meet-absorbing join-absorbing LattStr
for a, b being Element of L holds a [= a "\/" b
proof end;

theorem Th4: :: LATTICES:6
for L being non empty meet-commutative meet-absorbing LattStr
for a, b being Element of L holds a "/\" b [= a by Def8;

definition
let L be non empty meet-commutative meet-absorbing join-absorbing LattStr ;
let a, b be Element of L;
:: original: [=
redefine pred a [= b;
reflexivity
for a being Element of L holds (L,b1,b1) ;
end;

theorem :: LATTICES:7
for L being non empty join-associative \/-SemiLattStr
for a, b, c being Element of L st a [= b & b [= c holds
a [= c by Def5;

theorem Th6: :: LATTICES:8
for L being non empty join-commutative \/-SemiLattStr
for a, b being Element of L st a [= b & b [= a holds
a = b
proof end;

theorem Th7: :: LATTICES:9
for L being non empty meet-associative meet-absorbing join-absorbing LattStr
for a, b, c being Element of L st a [= b holds
a "/\" c [= b "/\" c
proof end;

theorem :: LATTICES:10
for L being Lattice st ( for a, b, c being Element of L holds ((a "/\" b) "\/" (b "/\" c)) "\/" (c "/\" a) = ((a "\/" b) "/\" (b "\/" c)) "/\" (c "\/" a) ) holds
L is distributive
proof end;

theorem Th9: :: LATTICES:11
for L being D_Lattice
for a, b, c being Element of L holds a "\/" (b "/\" c) = (a "\/" b) "/\" (a "\/" c)
proof end;

theorem Th10: :: LATTICES:12
for L being D_Lattice
for a, b, c being Element of L st c "/\" a = c "/\" b & c "\/" a = c "\/" b holds
a = b
proof end;

theorem :: LATTICES:13
for L being D_Lattice
for a, b, c being Element of L holds ((a "\/" b) "/\" (b "\/" c)) "/\" (c "\/" a) = ((a "/\" b) "\/" (b "/\" c)) "\/" (c "/\" a)
proof end;

registration
cluster non empty Lattice-like distributive -> modular for LattStr ;
coherence
for b1 being Lattice st b1 is distributive holds
b1 is modular
proof end;
end;

registration
let L be 0_Lattice;
let a be Element of L;
reduce (Bottom L) "\/" a to a;
reducibility
(Bottom L) "\/" a = a
proof end;
reduce (Bottom L) "/\" a to Bottom L;
reducibility
(Bottom L) "/\" a = Bottom L by Def16;
end;

theorem :: LATTICES:14
for L being 0_Lattice
for a being Element of L holds (Bottom L) "\/" a = a ;

theorem :: LATTICES:15
for L being 0_Lattice
for a being Element of L holds (Bottom L) "/\" a = Bottom L ;

theorem :: LATTICES:16
for L being 0_Lattice
for a being Element of L holds Bottom L [= a ;

registration
let L be 1_Lattice;
let a be Element of L;
reduce (Top L) "/\" a to a;
reducibility
(Top L) "/\" a = a
proof end;
reduce (Top L) "\/" a to Top L;
reducibility
(Top L) "\/" a = Top L by Def17;
end;

theorem :: LATTICES:17
for L being 1_Lattice
for a being Element of L holds (Top L) "/\" a = a ;

theorem :: LATTICES:18
for L being 1_Lattice
for a being Element of L holds (Top L) "\/" a = Top L ;

theorem :: LATTICES:19
for L being 1_Lattice
for a being Element of L holds a [= Top L
proof end;

definition
let L be non empty LattStr ;
let x be Element of L;
assume A1: L is complemented D_Lattice ;
func x ` -> Element of L means :Def21: :: LATTICES:def 21
it is_a_complement_of x;
existence
ex b1 being Element of L st b1 is_a_complement_of x by A1, Def19;
uniqueness
for b1, b2 being Element of L st b1 is_a_complement_of x & b2 is_a_complement_of x holds
b1 = b2 by A1, Th10;
end;

:: deftheorem Def21 defines ` LATTICES:def 21 :
for L being non empty LattStr
for x being Element of L st L is complemented D_Lattice holds
for b3 being Element of L holds
( b3 = x ` iff b3 is_a_complement_of x );

theorem Th18: :: LATTICES:20
for L being B_Lattice
for a being Element of L holds (a `) "/\" a = Bottom L
proof end;

theorem Th19: :: LATTICES:21
for L being B_Lattice
for a being Element of L holds (a `) "\/" a = Top L
proof end;

registration
let L be B_Lattice;
let a be Element of L;
reduce (a `) ` to a;
reducibility
(a `) ` = a
proof end;
end;

theorem :: LATTICES:22
for L being B_Lattice
for a being Element of L holds (a `) ` = a ;

theorem Th21: :: LATTICES:23
for L being B_Lattice
for a, b being Element of L holds (a "/\" b) ` = (a `) "\/" (b `)
proof end;

theorem :: LATTICES:24
for L being B_Lattice
for a, b being Element of L holds (a "\/" b) ` = (a `) "/\" (b `)
proof end;

theorem Th23: :: LATTICES:25
for L being B_Lattice
for a, b being Element of L holds
( b "/\" a = Bottom L iff b [= a ` )
proof end;

theorem :: LATTICES:26
for L being B_Lattice
for a, b being Element of L st a [= b holds
b ` [= a `
proof end;

:: missing, 2009.07.28, A.T.
definition
let L be non empty LattStr ;
let S be Subset of L;
attr S is initial means :Def22: :: LATTICES:def 22
 for p, q being Element of L st p [= q & q in S holds
p in S;
attr S is final means :Def23: :: LATTICES:def 23
 for p, q being Element of L st p [= q & p in S holds
q in S;
attr S is meet-closed means :: LATTICES:def 24
 for p, q being Element of L st p in S & q in S holds
p "/\" q in S;
attr S is join-closed means :: LATTICES:def 25
 for p, q being Element of L st p in S & q in S holds
p "\/" q in S;
end;

:: deftheorem Def22 defines initial LATTICES:def 22 :
for L being non empty LattStr
for S being Subset of L holds
( S is initial iff for p, q being Element of L st p [= q & q in S holds
p in S );

:: deftheorem Def23 defines final LATTICES:def 23 :
for L being non empty LattStr
for S being Subset of L holds
( S is final iff for p, q being Element of L st p [= q & p in S holds
q in S );

:: deftheorem defines meet-closed LATTICES:def 24 :
for L being non empty LattStr
for S being Subset of L holds
( S is meet-closed iff for p, q being Element of L st p in S & q in S holds
p "/\" q in S );

:: deftheorem defines join-closed LATTICES:def 25 :
for L being non empty LattStr
for S being Subset of L holds
( S is join-closed iff for p, q being Element of L st p in S & q in S holds
p "\/" q in S );

registration
let L be non empty LattStr ;
cluster [#] L -> non empty initial final ;
coherence
( [#] L is initial & [#] L is final & not [#] L is empty ) ;
end;

registration
let L be non empty LattStr ;
cluster non empty initial final for M3( bool the carrier of L);
existence
ex b1 being Subset of L st
( b1 is initial & b1 is final & not b1 is empty )
proof end;
cluster empty -> initial final for M3( bool the carrier of L);
coherence
for b1 being Subset of L st b1 is empty holds
( b1 is initial & b1 is final ) ;
end;

registration
let L be Lattice;
cluster non empty initial final for M3( bool the carrier of L);
existence
ex b1 being Subset of L st
( b1 is initial & b1 is final & not b1 is empty )
proof end;
end;

registration
let L be non empty meet-commutative meet-absorbing LattStr ;
cluster initial -> meet-closed for M3( bool the carrier of L);
coherence
for b1 being Subset of L st b1 is initial holds
b1 is meet-closed by Th4;
end;

registration
let L be non empty join-associative meet-commutative meet-absorbing join-absorbing LattStr ;
cluster final -> join-closed for M3( bool the carrier of L);
coherence
for b1 being Subset of L st b1 is final holds
b1 is join-closed by Th3;
end;

theorem :: LATTICES:27
for L being Lattice
for S being non empty initial final Subset of L holds S = [#] L
proof end;