The sub constructor allows subplanes of a projective or affine plane to be created. For classical planes, the SubfieldSubplane function is also provided.

sub<P | L> : Plane, List -> Plane

Given a plane P, construct the subplane of P generated by the points specified by L, where L is a list of one or more items of the following types:

• A point of P;
• A set or sequence of points of P;
• A subplane of P;
• A set or sequence of subplanes of P.

The set S of points defined by the list L must include a quadrangle if P is a projective plane and three non-collinear points if P is an affine plane. The function returns the smallest subplane of P containing S.

The plane obtained from the classical plane P by taking only those points of P which have all coordinates lying in F, where F must be a subfield of Field(P).

> K<w> := GF(4);
> P, V, L := ProjectivePlane(K);
> S := sub< P | [ V | [1, 0, 0], [0, 1, 0], [0, 0, 1], [1, w, 1] ] >;
> S: Maximal;
Projective Plane of order 2
Points: {@ ( 1 : 0 : 0 ), ( 0 : 1 : 0 ), ( 0 : 0 : 1 ), ( 1 : w : 0 ),
( 1 : 0 : 1 ), ( 1 : w : 1 ), ( 0 : 1 : w^2 ) @}
Lines:
    {( 0 : 1 : 0 ), ( 0 : 0 : 1 ), ( 0 : 1 : w^2 )},
    {( 1 : 0 : 0 ), ( 0 : 0 : 1 ), ( 1 : 0 : 1 )},
    {( 1 : 0 : 0 ), ( 0 : 1 : 0 ), ( 1 : w : 0 )},
    {( 1 : 0 : 0 ), ( 1 : w : 1 ), ( 0 : 1 : w^2 )},
    {( 0 : 1 : 0 ), ( 1 : 0 : 1 ), ( 1 : w : 1 )},
    {( 0 : 0 : 1 ), ( 1 : w : 0 ), ( 1 : w : 1 )},
    {( 1 : w : 0 ), ( 1 : 0 : 1 ), ( 0 : 1 : w^2 )}

> A := AffinePlane(4);
> S := SubfieldSubplane(A, GF(2));   
> S: Maximal;
Affine Plane AG(2, 2)
> S subset A;
true