On Graph Theory (-1974)

"I am not content with algebra, in that it yields neither the shortest proofs nor the most beautiful constructions of geometry. Consequently, in view of this, I consider that we need yet another kind of analysis, geometric or linear, which deals directly with position, as algebra deals with magnitude." (Gottfried Leibniz, [letter to Christiaan Huygens] 1670)

"A problem was posed to me about an island in the city of Königsberg, surrounded by a river spanned by seven bridges, and I was asked whether someone could traverse the separate bridges in a connected walk in such a way that each bridge is crossed only once. I was informed that hitherto no-one had demonstrated the possibility of doing this, or shown that it is impossible. This question is so banal, but seemed to me worthy of attention in that not geometry, nor algebra, nor even the art of counting was sufficient to solve it. In view of this, it occurred to me to wonder whether it belonged to the geometry of position, which Leibniz had once so much longed for. And so, after some deliberation, I obtained a simple, yet completely established, rule with whose help one can immediately decide for all examples of this kind, with any number of bridges in any arrangement, whether or not such a round trip is possible […]" (Leonard Euler, [letter to Giovanni Marinoni] 1736)

"There are seven bridges. If the problem could be reduced to numbers, why couldn’t I find a mathematical approach to solving it? It’s nothing to do with mathematics - it’s a purely logical problem, but that’s what intrigued me about it." (Leonhard Euler, [letter to Carl Leonhard Gottlieb Ehler, mayor of Danzig] 1736)

"Thus, you see, most noble Sir, how this type of solution bears little relationship to mathematics, and I do not understand why you expect a mathematician to produce it, rather than anyone else, for the solution is based on reason alone, and its discovery does not depend on any mathematical principle. Because of this, I do not know why even questions which bear so little relationship to mathematics are solved more quickly by mathematicians than by others." (Leonhard Euler, [letter to Carl Leonhard Gottlieb Ehler, mayor of Danzig] 1736)

"The theory of ramification is one of pure colligation, for it takes no account of magnitude or position; geometrical lines are used, but these have no more real bearing on the matter than those employed in genealogical tables have in explaining the laws of procreation." (James J Sylvester, "On Recent Discoveries in Mechanical Conversion of Motion", Proceedings of the Royal Institution of Great Britain, 1873-75)

"Some inkling of the nature of the difficulty of the question, unless its weak point be discovered and attacked, may be derived from the fact that a very small alteration in

one part of a map may render it necessary to recolor it throughout. After a somewhat arduous search, I have succeeded, suddenly, as might be expected, in hitting upon the weak point, which proved an easy one to attack. The result is, that the experience of the map makers has not deceived them, the maps they had to deal with, viz.: those drawn on a [sphere] can in every case be painted with four colors." (Alfred Kempe," How to Colour a Map with Four Colours", The American Journal of Mathematics No. 1, 1879)

"The Descriptive-Geometry Theorem that any map whatsoever can have its divisions

properly distinguished by the use of but four colors, from its generality and intangibility, seems to have aroused a good deal of interest a few years ago when the rigorous proof of it appeared to be difficult if not impossible, though no case of failure could be found. The present article does not profess to give a proof of this original Theorem; in fact its aims are so far rather destructive than constructive, for it will be shown that there is a defect in the now apparently recognized proof [of Kempe’s]" (Percy J Heawood, 1890)

"As a simple trick, the discrete can often be carried over into the continuous, in a way suitable for practical purposes, by making a graph of the discrete, with the values shown as separate points. It is then easy to see the form that the changes will take if the points were to become infinitely numerous and close together." (W Ross Ashby, "An Introduction to Cybernetics", 1956)

"A common objection to the use of mathematics in the social sciences is that the information available may only be qualitative, not quantitative. There are, however, several branches of mathematics that deal effectively with qualitative information. A very good example is graph theory." (John G Kemeny, "The Social Sciences Call on Mathematics", The Mathematical Sciences: A Collection of Essays, 1969)

"Graph theory, a special tool borrowed from topology, has now been used to reduce even quite complicated chemical structures to a chain of numbers so that a computer can analyze them." (George A W Boehm, "The Mathematical Sciences: A Collection of Essays", 1969)