A ring in graph theory
W. T. Tutte · Mathematical Proceedings of the Cambridge Philosophical Society · 1947
We call a point set in a complex K a 0-cell if it contains just one point of K, and a 1-cell if it is an open arc. A set L of 0-cells and 1-cells of K is called a linear graph on K if (i) no two members of L intersect, (ii) the union of all the members of L is K, (iii) each end-point of a 1-cell of L is a 0-cell of L and (iv) the number of 0-cells and 1-cells of L is finite and not 0.