Journal of Formalized Mathematics
Volume 2, 1990
University of Bialystok
Copyright (c) 1990 Association of Mizar Users

### A Projective Closure and Projective Horizon of an Affine Space

by
Henryk Oryszczyszyn, and
Krzysztof Prazmowski

MML identifier: AFPROJ
[ Mizar article, MML identifier index ]

```environ

vocabulary DIRAF, AFF_4, INCSP_1, AFF_1, ANALOAF, PARSP_1, BOOLE, SETFAM_1,
EQREL_1, RELAT_1, RELAT_2, ANPROJ_1, INCPROJ, MCART_1, ANPROJ_2, AFF_2,
VECTSP_1, AFPROJ, PARTFUN1;
notation TARSKI, XBOOLE_0, ZFMISC_1, SUBSET_1, EQREL_1, RELSET_1, RELAT_1,
RELAT_2, PARTFUN1, MCART_1, STRUCT_0, ANALOAF, DIRAF, AFF_1, AFF_4,
PAPDESAF, INCSP_1, INCPROJ;
constructors DOMAIN_1, AFF_2, TRANSLAC, EQREL_1, RELAT_2, AFF_1, AFF_4,
INCPROJ, XBOOLE_0;
clusters INCPROJ, RELSET_1, STRUCT_0, SUBSET_1, XBOOLE_0, ZFMISC_1, PARTFUN1;
requirements SUBSET, BOOLE;

begin

reserve AS for AffinSpace;
reserve A,K,M,X,Y,Z,X',Y' for Subset of AS;
reserve zz for Element of AS;
reserve x,y for set;

:: the projective closure of an affine space. We begin with some evident
:: properties of planes in affine planes.

theorem :: AFPROJ:1
AS is AffinPlane & X=the carrier of AS implies X is_plane;

theorem :: AFPROJ:2
AS is AffinPlane & X is_plane implies X = the carrier of AS;

theorem :: AFPROJ:3
AS is AffinPlane & X is_plane & Y is_plane implies X=Y;

theorem :: AFPROJ:4
X=the carrier of AS & X is_plane implies AS is AffinPlane;

theorem :: AFPROJ:5
not A // K & A '||' X & A '||' Y & K '||' X & K '||' Y & A is_line &
K is_line & X is_plane & Y is_plane implies X '||' Y;

theorem :: AFPROJ:6
X is_plane & A '||' X & X '||' Y implies A '||' Y;

:: Next we distinguish two sets, one consisting of the lines, the second
:: consisting of the planes of a given affine space and we consider two
:: equivalence relations defined on each of these sets; theses relations
:: are in fact the relation of parallelity restricted to suitable area.
:: Their equivalence classes are called directions (of lines and planes,
:: respectively); they are intended to be considered as new objects which
:: extend the given affine space to a projective space.

definition let AS; func AfLines(AS) -> Subset-Family of AS equals
:: AFPROJ:def 1
{A: A is_line}; end;

definition let AS; func AfPlanes(AS) -> Subset-Family of AS equals
:: AFPROJ:def 2
{A: A is_plane}; end;

theorem :: AFPROJ:7
for x holds (x in AfLines(AS) iff ex X st x=X & X is_line);

theorem :: AFPROJ:8
for x holds (x in AfPlanes(AS) iff ex X st x=X & X is_plane);

definition let AS;
func LinesParallelity(AS) -> Equivalence_Relation of AfLines(AS)
equals
:: AFPROJ:def 3
{[K,M]: K is_line & M is_line & K '||' M}; end;

definition let AS;
func PlanesParallelity(AS) -> Equivalence_Relation of AfPlanes(AS)
equals
:: AFPROJ:def 4
{[X,Y]: X is_plane & Y is_plane & X '||' Y}; end;

definition let AS,X such that X is_line; func LDir(X) -> Subset of AfLines(AS)
equals
:: AFPROJ:def 5
Class(LinesParallelity(AS),X);
end;

definition let AS,X such that
X is_plane; func PDir(X) -> Subset of AfPlanes(AS)
equals
:: AFPROJ:def 6
Class(PlanesParallelity(AS),X);
end;

theorem :: AFPROJ:9
X is_line implies for x holds x in LDir(X) iff ex Y st x=Y
& Y is_line & X '||' Y;

theorem :: AFPROJ:10
X is_plane implies for x holds x in PDir(X) iff ex Y st x=Y
& Y is_plane & X '||' Y;

theorem :: AFPROJ:11
X is_line & Y is_line implies (LDir(X)=LDir(Y) iff X // Y);

theorem :: AFPROJ:12
X is_line & Y is_line implies (LDir(X)=LDir(Y) iff X '||' Y);

theorem :: AFPROJ:13
X is_plane & Y is_plane implies (PDir(X)=PDir(Y) iff X '||' Y);

definition let AS; func Dir_of_Lines(AS) -> non empty set
equals
:: AFPROJ:def 7
Class LinesParallelity(AS); end;

definition let AS; func Dir_of_Planes(AS) -> non empty set
equals
:: AFPROJ:def 8
Class PlanesParallelity(AS); end;

theorem :: AFPROJ:14
for x holds x in Dir_of_Lines(AS) iff ex X st x=LDir(X)
& X is_line;

theorem :: AFPROJ:15
for x holds x in Dir_of_Planes(AS) iff ex X st x=PDir(X)
& X is_plane;

:: The point is to guarantee that the classes of new objects consist of really
:: new objects. Clearly the set of directions of lines and the set of affine
:: points do not intersect. However we cannot claim, in general, that the set
:: of affine lines and the set of directions of planes do not intersect; this
:: is evidently true only in the case of affine planes. Therefore we have to
:: modify (slightly) a construction of the set of lines of the projective
:: closure of affine space, when compared with (naive) intuitions.

theorem :: AFPROJ:16
the carrier of AS misses Dir_of_Lines(AS);

theorem :: AFPROJ:17
AS is AffinPlane implies AfLines(AS) misses Dir_of_Planes(AS);

theorem :: AFPROJ:18
for x holds (x in [:AfLines(AS),{1}:] iff
ex X st x=[X,1] & X is_line);

theorem :: AFPROJ:19
for x holds (x in [:Dir_of_Planes(AS),{2}:] iff
ex X st x=[PDir(X),2] & X is_plane);

definition let AS;
func ProjectivePoints(AS) -> non empty set equals
:: AFPROJ:def 9
(the carrier of AS) \/ Dir_of_Lines(AS);
end;

definition let AS;
func ProjectiveLines(AS) -> non empty set equals
:: AFPROJ:def 10
[:AfLines(AS),{1}:] \/ [:Dir_of_Planes(AS),{2}:];
end;

definition let AS;
func Proj_Inc(AS) -> Relation of ProjectivePoints(AS),ProjectiveLines(AS)
means
:: AFPROJ:def 11
for x,y holds [x,y] in it iff
(ex K st K is_line & y=[K,1] & (x in the carrier of AS & x in K or
x = LDir(K))) or
(ex K,X st K is_line & X is_plane & x=LDir(K) & y=[PDir(X),2] & K '||' X); end;

definition let AS; func
Inc_of_Dir(AS) -> Relation of Dir_of_Lines(AS),Dir_of_Planes(AS) means
:: AFPROJ:def 12
for x,y holds [x,y] in it iff ex A,X st x=LDir(A) & y=PDir(X) &
A is_line & X is_plane & A '||' X; end;

definition let AS; func
IncProjSp_of(AS) -> strict IncProjStr
equals
:: AFPROJ:def 13
IncProjStr (# ProjectivePoints(AS),
ProjectiveLines(AS), Proj_Inc(AS) #);
end;

definition let AS; func ProjHorizon(AS) -> strict IncProjStr equals
:: AFPROJ:def 14
IncProjStr (#Dir_of_Lines(AS),
Dir_of_Planes(AS), Inc_of_Dir(AS) #);
end;

theorem :: AFPROJ:20
for x holds (x is POINT of IncProjSp_of(AS)
iff (x is Element of AS or ex X st x=LDir(X) & X is_line));

theorem :: AFPROJ:21
x is Element of the Points of ProjHorizon(AS) iff
ex X st x=LDir(X) & X is_line;

theorem :: AFPROJ:22
x is Element of the Points of ProjHorizon(AS) implies
x is POINT of IncProjSp_of(AS);

theorem :: AFPROJ:23
for x holds (x is LINE of IncProjSp_of(AS) iff
ex X st (x=[X,1] & X is_line or x=[PDir(X),2] & X is_plane));

theorem :: AFPROJ:24
x is Element of the Lines of ProjHorizon(AS) iff
ex X st x=PDir(X) & X is_plane;

theorem :: AFPROJ:25
x is Element of the Lines of ProjHorizon(AS) implies
[x,2] is LINE of IncProjSp_of(AS);

reserve x,y,z,t,u,w for Element of AS;
reserve K,X,Y,Z,X',Y' for Subset of AS;
reserve a,b,c,d,p,q,r,p' for POINT of IncProjSp_of(AS);
reserve A for LINE of IncProjSp_of(AS);

theorem :: AFPROJ:26
x=a & [X,1]=A implies (a on A iff X is_line & x in X);

theorem :: AFPROJ:27
x=a & [PDir(X),2]=A & X is_plane implies not a on A;

theorem :: AFPROJ:28
a=LDir(Y) & [X,1]=A & Y is_line & X is_line implies (a on A iff Y '||' X);

theorem :: AFPROJ:29
a=LDir(Y) & A=[PDir(X),2] & Y is_line & X is_plane implies (a on A iff
Y '||' X);

theorem :: AFPROJ:30
X is_line & a=LDir(X) & A=[X,1] implies a on A;

theorem :: AFPROJ:31
X is_line & Y is_plane & X c= Y & a=LDir(X) & A=[PDir(Y),2]
implies a on A;

theorem :: AFPROJ:32
Y is_plane & X c= Y & X' // X & a=LDir(X') & A=[PDir(Y),2]
implies a on A;

theorem :: AFPROJ:33
A=[PDir(X),2] & X is_plane & a on A
implies a is not Element of AS;

theorem :: AFPROJ:34
A=[X,1] & X is_line & p on A & p is not Element of AS
implies p=LDir(X);

theorem :: AFPROJ:35
A=[X,1] & X is_line & p on A & a on A & a<>p &
not p is Element of
the carrier of AS implies a is Element of AS;

theorem :: AFPROJ:36
for a being Element of the Points of ProjHorizon(AS),A being Element
of the Lines of ProjHorizon(AS) st a=LDir(X) & A=PDir(Y) & X is_line
& Y is_plane holds (a on A iff X '||' Y);

theorem :: AFPROJ:37
for a being Element of the Points of ProjHorizon(AS),a' being Element of
the Points of IncProjSp_of(AS),A being Element of the Lines of
ProjHorizon(AS),A' being LINE of IncProjSp_of(AS)
st a'=a & A'=[A,2] holds (a on A iff a' on A');

reserve A,K,M,N,P,Q for LINE of IncProjSp_of(AS);

theorem :: AFPROJ:38
for a,b being Element of the Points of ProjHorizon(AS),
A,K being Element
of the Lines of ProjHorizon(AS) st a on A & a on K & b on A & b on K holds
a=b or A=K;

theorem :: AFPROJ:39
for A being Element of the Lines of ProjHorizon(AS) ex a,b,c
being Element of the Points of ProjHorizon(AS) st a on A & b on A & c on A &
a<>b & b<>c & c <>a;

theorem :: AFPROJ:40
for a,b being Element of the Points of ProjHorizon(AS)
ex A being
Element of the Lines of ProjHorizon(AS) st a on A & b on A;

theorem :: AFPROJ:41
for x,y being Element of the Points of ProjHorizon(AS), X being Element
of the Lines of IncProjSp_of(AS) st x<>y & [x,X] in the Inc of
IncProjSp_of(AS) & [y,X] in the Inc of IncProjSp_of(AS) ex
Y being Element of the Lines of ProjHorizon(AS) st X=[Y,2];

theorem :: AFPROJ:42
for x being POINT of IncProjSp_of(AS),X being
Element of the Lines of ProjHorizon(AS) st [x,[X,2]] in the Inc of
IncProjSp_of(AS) holds x is Element of the Points of ProjHorizon(AS);

theorem :: AFPROJ:43
Y is_plane & X is_line & X' is_line & X c= Y & X' c= Y &
P=[X,1] & Q=[X',1] implies ex q st q on P & q on Q;

theorem :: AFPROJ:44
for a,b,c,d,p being Element of the Points of ProjHorizon(AS),M,N,P,Q
being Element of the Lines of ProjHorizon(AS) st
a on M & b on M & c on N & d on N & p on M &
p on N & a on P & c on P & b on Q & d on Q & not p on P & not p on Q & M<>N
ex q being Element of
the Points of ProjHorizon(AS) st q on P & q on Q;

definition let AS;
cluster IncProjSp_of(AS) -> partial linear up-2-dimensional
up-3-rank Vebleian;
end;

definition
cluster strict 2-dimensional IncProjSp;
end;

definition let AS be AffinPlane;
cluster IncProjSp_of(AS) -> 2-dimensional; end;

theorem :: AFPROJ:45
IncProjSp_of(AS) is 2-dimensional implies AS is AffinPlane;

theorem :: AFPROJ:46
AS is not AffinPlane implies ProjHorizon(AS) is IncProjSp;

theorem :: AFPROJ:47
ProjHorizon(AS) is IncProjSp implies AS is not AffinPlane;

theorem :: AFPROJ:48
for M,N being Subset of AS, o,a,b,c,a',b',c' being Element of
the carrier of AS st M is_line & N is_line & M<>N &
o in M & o in N & o<>a & o<>a' & o<>b & o<>b' & o<>c & o<>c' & a in M &
b in M & c in M & a' in N & b' in N & c' in N &
a,b' // b,a' & b,c' // c,b' & (a=b or b=c or a=c) holds a,c' // c,a';

theorem :: AFPROJ:49
IncProjSp_of(AS) is Pappian implies AS is Pappian;

theorem :: AFPROJ:50
for A,P,C being Subset of AS, o,a,b,c,a',b',c' being Element of
the carrier of AS st o in A & o in P & o in C &
o<>a & o<>b & o<>c & a in A & a' in A & b in P & b' in P &
c in C & c' in C & A is_line & P is_line & C is_line &
A<>P & A<>C & a,b // a',b' & a,c // a',c' & (o=a' or a=a') holds
b,c // b',c';

theorem :: AFPROJ:51
IncProjSp_of(AS) is Desarguesian implies AS is
Desarguesian;

theorem :: AFPROJ:52
IncProjSp_of(AS) is Fanoian implies AS is Fanoian;
```