There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Planar Graphs. Graphs on Higher Surfaces. Degrees. Critical Graphs. The Conjectures of Hadwiger and Hajos. Sparse Graphs. Perfect Graphs. Geometric and Combinatorial Graphs. Algorithms. Constructions. Edge Colorings. Orientations and Flows. Chromatic Polynomials. Hypergraphs. Infinite Chromatic Graphs. Miscellaneous Problems. Indexes.

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Graph Coloring Problems

A sequence a,, a,, . . . , a, of real numbers is said to be unimodal if for some 0 s j _c n we have a, 5 a , 5 . . 5 ai 2 a,,, 2 . . 2 a,, and is said to be logarithmically concave (or log-concave for short) if a: 2 a, ,a ,+ , for all 1 5 i 5 n 1. Clearly a log-concave sequence of positive terms is unimodal. Let us say that the sequence a,, a,, . . . , a, has no internal zeros if there do not exist integers 0 5 i < j < k 5 n satisfying a, f 0, a, = 0, ak # 0. Then in fact a nonnegative log-concave sequence with no internal zeros is unimodal. The sequence a,, a , , . . . , a, is called symmetric if a, = a,-, for 0 5 i 5 n. Thus a symmetric unimodal sequence a,, a, , . . . , a,, has its maximum at the middle term ( n even) or middle two terms (n odd). We also say that a polynomial a, + a,4 +. . + an$ has a certain property (such as unimodal, log-concave, or symmetric) if its sequence a,, a,, . . . , a, of coefficients has that property. Our object here is to survey the surprisingly rich variety of methods for showing that a sequence is log-concave or unimodal. For each method we will give examples of its applicability to combinatorially defined sequences that arise naturally from problems in algebra, combinatorics, and geometry. We make no attempt, however, to give a comprehensive account of all work done in this area.

Log‐Concave and Unimodal Sequences in Algebra, Combinatorics, and Geometry

In One Line and Close. Permutations as Linear Orders. Descents Alternating Runs Alternating Subsequences In One Line and Anywhere. Permutations as Linear Orders. Inversions. Inversions Inversion in Permutations of Multisets In Many Circles. Permutations as Products of Cycles. Decomposing a Permutation into Cycles Type and Stirling Numbers Cycle Decomposition versus Linear Order Permutations with Restricted Cycle Structure In Any Way but This. Pattern Avoidance. The Basics. The Notion of Pattern Avoidance Patterns of Length Three Monotone Patterns Patterns of Length Four The Proof of the Stanley-Wilf Conjecture In This Way but Nicely. Pattern Avoidance. Follow-Up. Polynomial Recurrences Containing a Pattern Many Times Containing a Pattern a Given Number of Times Mean and Insensitive. Random Permutations. The Probabilistic Viewpoint Expectation Variance and Standard Deviation An Application: Longest Increasing Subsequences Permutations versus Everything Else. Algebraic Combinatorics of Permutations. The Robinson-Schensted-Knuth Correspondence Posets of Permutations Simplicial Complexes of Permutations Get Them All. Algorithms and Permutations. Generating Permutations Stack Sorting Permutations Variations of Stack Sorting How Did We Get Here? Permutations as Genome Rearrangements. Introduction Block Transpositions Block Interchanges Block Transpositions Revisited Solutions to Odd-Numbered Exercises References List of Frequently Used Notation Index Exercises, Problems, and Problem Solutions appear at the end of each chapter.

Combinatorics of Permutations

I.- The basics.- Bruhat order.- Weak order and reduced words.- Roots, games, and automata.- II.- Kazhdan-Lusztig and R-polynomials.- Kazhdan-Lusztig representations.- Enumeration.- Combinatorial Descriptions.

Combinatorics of Coxeter Groups

Un grand nombre de suites d'interet combinatoire sont unimodales ou log-concaves. Il s'agit de proprietes difficiles a montrer et non conservees par des transformations lineaires, en general. On peut affiner ces deux notions pour definir les suites de frequences de Polya qui possedent des proprietes plus convenables. On montre des applications combinatoires diverses, telle que la coloration de graphes, de ces suites

Unimodal Log Concave and Polya Frequency Sequences in Combinatorics

We study the polynomials obtained by enumerating a finite Coxeter group by number of descents. The type A case gives rise to the familiar Eulerian polynomials, while the B and D cases provided two new q -analogues. Various recursion relations, generating functions and unimodality properties are derived, which generalize and unify earlier results of Dolgachev, Lunts, Stanley and Stembridge.

Francesco Brenti

Papers

Combinatorics of Coxeter Groups

Unimodal Log Concave and Polya Frequency Sequences in Combinatorics

q -Eulerian polynomials arising from Coxeter groups

Combinatorics and total positivity

Descent Numbers and Major Indices for the Hyperoctahedral Group