> < ^ Date: Thu, 22 Dec 1994 16:22:00 +0100 (WET)
> < ^ From: Thomas Breuer <Thomas.Breuer@Math.RWTH-Aachen.DE >
^ Subject: Vector Spaces in GAP-3.4

Dear Mrs. and Mr. Forum,

today 'gap-trouble' got a message about a problem with vector spaces in GAP.
Perhaps it is useful to tell this story to you, too.

Suppose you have a (row) vector space <V> and a list of vectors that form
a basis of <V>, and you want to compute the coefficients of arbitrary
vectors in <V> with respect to this basis.
In GAP-3.4 there is a solution for this, contrary to previous versions.

In GAP-3.3 only very special vector space bases were supported.
If the user wanted to define a basis for <V> this was done
by 'AddBase( <V>, <vectors> )', and if <vectors> was not a matrix in upper
triangular form then the basis could not be used for the computation of
coefficients.

In GAP-3.4 another mechanism is used to deal with bases of row spaces.
Such a basis is now a record constructed by 'Basis( <V>, <vectors> )'.
The basis knows about the space, the basis vectors, and how to compute
coefficients.
This means the basis is asked for the coefficients of a vector, not the
space.
The main differences between the mechanisms in GAP-3.3 and GAP-3.4 are
that in GAP-3.4 arbitrary row space bases can be defined, and that one
can deal with several bases for the same space.

Here is an example.

```gap> # Construct the space.
gap> vectors:= [ [ 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 ],
>                [ 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0 ],
>                [ 1, 1, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0 ],
>                [ 1, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0 ],
>                [ 0, 1, 0, 0, 1, 1, 1, 0, 1, 0, 0, 1 ],
>                [ 0, 0, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1 ] ] * Z(3)^0;;
gap> golay:= VectorSpace( vectors, GF(3) );
RowSpace( GF(3),
[ [ Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0,
Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0 ],
[ Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, Z(3)^0, 0*Z(3), 0*Z(3),
0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3) ],
[ Z(3)^0, Z(3)^0, Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0, Z(3)^0,
Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ],
[ Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0, Z(3)^0, 0*Z(3), Z(3)^0,
Z(3)^0, Z(3)^0, 0*Z(3), 0*Z(3) ],
[ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3), Z(3)^0, Z(3)^0, Z(3)^0, 0*Z(3),
Z(3)^0, 0*Z(3), 0*Z(3), Z(3)^0 ],
[ 0*Z(3), 0*Z(3), Z(3)^0, Z(3)^0, 0*Z(3), Z(3)^0, 0*Z(3), Z(3)^0,
Z(3)^0, 0*Z(3), 0*Z(3), Z(3)^0 ] ] )
gap> Dimension( golay );
6
gap>
gap> # Define the basis.
gap> b:= Basis( golay, golay.generators );;
gap>
gap> # Compute coefficients.
gap> List( golay.generators, x -> Coefficients( b, x ) );
[ [ Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3) ],
[ 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3) ],
[ 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3), 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3), 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0, 0*Z(3) ],
[ 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ] ]
gap>
```

Kind regards
Thomas Breuer

> < [top]