With E as some index of environmental quality and C1 and C2 as two ‘ordinary’ commodities, consider the following utility functions in regard to whether C1 is non-essential and whether C1 and E are weak complements:

(a) U = E α + C1 β + C2 δ

(b) U = E α
C1 β
C2 δ

(c) U = E α
C1 β + C2 δ