Truncations

Let \(\Gamma(X,\sim,t,I)\) be an incidence geometry and let \(J\) be a subset of \(I\). Then the J–truncation of \(\Gamma\) is the geometry whose set of elements is \(t^{-1}(J)\), together with the restricted type function and incidence relation.

Let \(J \subseteq I\). The J–truncation of the coset geometry \(\Gamma(G;(G_i)_{i \in I})\) is the coset geometry \(\Gamma( G; (G_j)_{j\in J})\).

Truncation(D, t): IncGeom, Set -> IncGeom

Given an incidence geometry \(D\) and \(t\) a subset of the set of types of \(D\), return the t–truncation of \(D\) as an incidence geometry.

Truncation(C, t): CosetGeom, Set -> CosetGeom

Given a coset geometry \(C\) and \(t\) a subset of the set of types of \(C\), return the t–truncation of \(C\) as a coset geometry.