MAGMA Computational Algebra System

Shadows

Let Γ(X, ~, t, I) be an incidence geometry. Let J be a subset of I and let F be a flag of Γsuch that t(F) ∩J = emptyset. The J-shadow of the flag F, denoted by σ_J(F), is the set of flags of type J in the residue of the flag F.

Shadow(D, I, F) : IncGeom, Set, Set -> SetIndx

Given an incidence geometry D, a subset I of the set of types of D and a flag F of D, return the I-shadow of the flag F as an indexed set of subsets of points of D.