:: deftheorem Def6 defines eta POLYFORM:def 6 :
for p being polyhedron
for k being Integer
for b₃ being incidence-matrix of ((k - 1) -polytopes p),(k -polytopes p) holds
( b₃ = eta (p,k) iff ( ( k < 0 implies b₃ = {} ) & ( k = 0 implies b₃ = [:{{}},(0 -polytopes p):] --> (1. Z_2) ) & ( 0 < k & k < dim p implies b₃ = the IncidenceF of p . k ) & ( k = dim p implies b₃ = [:(((dim p) - 1) -polytopes p),{p}:] --> (1. Z_2) ) & ( k > dim p implies b₃ = {} ) ) );