Generalized Eulerian Numbers
JOURNAL OF COMBINATORIALTHEORY, Series A 32, 195-215 (1982) Generalized Eulerian Numbers D. H. LEHMER Department of Mathematics, University of Cal$ornia, Berkeley, California 94720 Received March 17, 1981 DEDICATED TO PROFESSOR MARSHALL HALL, JR., ON THE OCCASION OF HIS RETIREMENT 1. INTRODUCTION The Eulerian numbers (not to be confused with the Euler numbers E,) were discovered by Euler in 1755 [I]. Since then, they have been rediscovered, redefined, generalized and used by many authors [2-151. Explicitly they can be defined for 0 < k < n by k-l A(n,k)= 1 (-1)’ (k -j)“. j=O Euler wrote g+ f z-z,(x) P/n! n=O and (x - 1)” H,(x) = i A@, k) xk-‘. &=I This can be expressed by the generating function l-u = 1 + 2 f A@, k) ukt”/n! 1 - u exp{t(l - u)} &=I n=l from which the recurrence relation A@+ l,k)=(n-k+2)A(n,k- l)+kA(k,t) (3) is easily derived (see, for example, Comtet [ 15, Vol. 1, pp. 63-641. 195 0097-3 165/82/020195-21$02.00/O Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved. 196 D. H. LEHMER The connections between A(n, k) and the polynomials of Bernoulli and Euler, such as ml(x)-B,(O)) = ;:r (X+;-l)A(n-l,k), were among the first discoveries made. Later Eulerian Numbers began to occur in combinatorial problems and A@, k) is best remembered as the number of permutations 1 2 )...) n $l), 742) ,***,70) that have precisely k - 1 “rises,” where a rise is a pair of consecutive marks n(i), n(i + 1) with n(i) < n(i + 1).
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