[GAP Forum] How to obtain the incidence matrix of a partial geometry?
Sven Reichard
sven.reichard at tu-dresden.de
Wed Jan 21 12:51:03 GMT 2015
Hello Felix,
the geometry is returned as an incidence graph in GRAPE format. For each
vertex x of that graph, Adjacency(haemers, x) will give you its
neighbours. From that you can reconstruct the adjacency matrix if you want.
For example with the following function:
AdjacencyMatrix := function ( gamma )
local result, x, y;
result := NullMat( gamma.order, gamma.order );
for x in [ 1 .. gamma.order ] do
for y in Adjacency( gamma, x ) do
result[x][y] := 1;
od;
od;
return result;
end
However it could be worth your while working with the graph format as it is.
"delta" just refers to the incidence graph. PartialLinearSpaces returns
a list of those.
Hope this helps,
Sven Reichard
Institut für Algebra
TU Dresden
On 01/21/2015 01:33 PM, Felix Goldberg wrote:
> Hello all,
>
> I am running the code in the example in Section 9.2 of the GRAPE manual (
> http://www.maths.qmul.ac.uk/~leonard/grape/manual/CHAP009.htm) , which
> generates the Haemers partial geometry pg(4,17,2).
>
> All works well but I cannot understand where exactly the incidence matrix
> is stored and how to access it. The manual (referred to above) says that
> there is a *delta* associated to the geometry output by the function
> PartialLinearSpaces
> but I can't find it.
>
> I tried to run RecNames on the output (the variable called *haemers*) and
> got this:
>
> [ "names", "group", "order", "representatives", "isSimple", "isGraph",
> "schreierVector", "adjacencies" ]
>
> No sign of *delta* and apparently no incidence matrix.
>
> Any help will be greatly appreciated.
>
> Thanks,
> Felix
>
>
>
>
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