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Enumeration of Two Particular Sets of Minimal Permutations

Journal of Integer Sequences, Vol. 18 (2015), Article 15.10.2

Enumeration of Two Particular Sets of Minimal Permutations


Stefano Bilotta, Elisabetta Grazzini, and Elisa Pergola
Dipartimento di Matematica e Informatica "Ulisse Dini"
Università di Firenze
Viale G. B. Morgagni 65
50134 Firenze
Italy

Abstract:

Minimal permutations with d descents and size d + 2 have a unique ascent between two sequences of descents. Our aim is the enumeration of two particular sets of these permutations. The first set contains the permutations having d + 2 as the top element of the ascent. The permutations in the latter set have 1 as the last element of the first sequence of descents and are the reverse-complement of those in the other set. The main result is that these sets are enumerated by the second-order Eulerian numbers.


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(Concerned with sequences A000918 A005803.)


Received April 29 2015; revised version received August 25 2015. Published in Journal of Integer Sequences, September 8 2015.


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