theorem 7.5 is due to Herbret E. Robbins (1915-2001). The paper in which this theorem appears, titled "A theorem on graphs, with an application to a problem of traffic control", was published in 1939 in the American Mathematical Monthly, only a year after he received his Ph.D. form Harvard University in topology, under the direction of Hassler Whitney. This was only Robbins's second publication of what was to become a long and impressive list. Also, in 1939 at age 24, Robbins began work on the classic book What Is Mathematics ? with Richard Courant. this book has been classified by Robbins as more of a literary work than a scientific work. A few years later robbins became interested in and devoted his research to statistical analysis, in which he made major contributions. He spent mamy year as a Professor of Mathematical Statistics at Columbia University. In 1958 he had a doctoral student, Herbert Wilf, who has made major contributions to combinatorics and graph theory. Indeed, Wilf was a recipient of an Euler Medal in 2002 from the Institute of Combinatorics and Its Applications