Abstract
Generally speaking, Combinatorics deals with two different kinds of problems: those in which we want to count the number of distinct ways of making a certain choice and those in which we want to make sure that some configuration does appear. In order to solve a problem of the first kind above, we employ, among others, the counting techniques discussed in the previous chapters. For the second kind of problem, up to this moment we have not developed any idea that could be systematically used. It is our purpose in this chapter to remedy this state of things. To this end, we start by discussing the famous pigeonhole’s principle of Dirichlet, along with several interesting examples. Then we move on to some applications of the principle of mathematical induction to the existence of configurations. The chapter continues with the study of partial order relations, exploring Mirsky’s theorem on the relation between chains and anti-chains. We close the chapter by explaining how, in some situations, the search for an adequate invariant or a semi-invariant can give the final outcome of certain seemingly random algorithms.
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- 2.
Former name of the Russian city of Saint Petersburg.
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James Sylvester , English mathematician, and Tibor Gallai , Hungarian mathematician, both of the twentieth century.
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Nicolaas de Bruijn , Dutch mathematician of the twentieth century.
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After Leon Mirsky , Russian mathematician of the twentieth century.
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After Robert Dilworth , American mathematician of the twentieth century.
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Formally, an algorithm is a finite sequence of well defined operations that, once performed on some (more or less) arbitrary set of data (called the input of the algorithm), furnish a definite result, called the outcome or output of the algorithm. On the other hand, each such performance of the algorithm is generally referred to as an iteration of it. We shall encounter algorithms several times along these notes.
References
A. Caminha, An Excursion Through Elementary Mathematics I - Real Numbers and Functions (Springer, New York, 2017)
R. Dilworth, A decomposition theorem for partially ordered sets. Ann. Math. 51, 161–166 (1950)
P. Erdös, E. Szekeres, A combinatorial problem in geometry. Compos. Math. 2, 463–470 (1935)
F. Galvin, A proof of Dilworth’s chain decomposition theorem. Am. Math. Monthly 101, 352–353 (1994)
L. Mirsky, A dual of Dilworth’s decomposition theorem. Am. Math. Monthly. 78, 876–877 (1971)
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Caminha Muniz Neto, A. (2018). Existence of Configurations. In: An Excursion through Elementary Mathematics, Volume III. Problem Books in Mathematics. Springer, Cham. https://doi.org/10.1007/978-3-319-77977-5_4
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DOI: https://doi.org/10.1007/978-3-319-77977-5_4
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