Category Archives: combinatorics

Counting Poker Hands

For this post I’m going to go through a classic exercise in combinatorics and probability; namely, proving that the standard ranking of poker hands is correct. First, here are the standard poker hands, in ranked order. Straight flush: Five cards of … Continue reading

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Two Methods for Proving an Alternating Binomial Identity

Recently I gave an optional homework problem in which I asked students to prove the following binomial identity: . (Here, I’m using the Iverson bracket notation in which if P is true and if P is false.) I intended for students to … Continue reading

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A Proof of Dobinski’s Formula for Bell Numbers

Dobinski’s formula entails the following infinite series expression for the nth Bell number : In this post we’ll work through a proof of Dobinski’s formula.  We’ll need four formulas: The Maclaurin series for : . The formula for converting normal powers to falling … Continue reading

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Integrality of the Catalan Numbers via Kummer’s Theorem

Why is the nth Catalan number, , an integer?  If you know one of its combinatorial interpretations, then the answer is clear, but how do you get integrality strictly from this formula?  In this post I’m going to discuss how one can … Continue reading

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Tiling Proofs for the Sum of Even or Odd-Indexed Binomial Coefficients

Two basic identities for the binomial coefficients are contained in the equation .   There are multiple ways to prove these identities (even if you restrict yourself to combinatorial proofs).  In this post I’m going to discuss a tiling or … Continue reading

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Combinatorial Proofs of Two Hagen-Rothe Identities in Concrete Mathematics

There are two binomial coefficient identities in Concrete Mathematics that have bothered me for years:  They look they should have combinatorial proofs, but I couldn’t for the life of me think of what those might be.  (In Concrete Mathematics they are … Continue reading

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Vandermonde’s Identity from the Generalized Product Rule

Vandermonde’s Identity, is one of the more famous identities involving the binomial coefficients.  A standard way to prove it is with the following combinatorial argument. How many ways are there to choose a committee of size r from a group of n men … Continue reading

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