Voltage-Graphic Matroids

From an integer valued function f we obtain, in a natural way, a matroid Mf on the domain of f. We show that the class M of matroids so obtained is closed under restriction, contraction, duality, truncation and elongation, but not under direct sum. We give an excluded-minor characterisation of M and show that M consists precisely of those transversal matroids with a presentation in which the sets in the presentation are nested. Finally, we show that on an n-set there are exactly 2n members of M.