tailieunhanh - Báo cáo toán hoc:" q-Counting descent pairs with prescribed tops and bottoms"

Tuyển tập các báo cáo nghiên cứu khoa học ngành toán học tạp chí toán học quốc tế đề tài: q-Counting descent pairs with prescribed tops and bottoms. | q-Counting descent pairs with prescribed tops and bottoms John Haiư Jeffrey Liese Jeffrey B. Remmel Submitted Oct 12 2008 Accepted Aug 25 2009 Published Aug 31 2009 Mathematics Subject Classification 05A05 05A15 Abstract Given sets X and Y of positive integers and a permutation ơ ơ1ơ2 ơn E Sn an X Y -descent of ơ is a descent pair ơi ơị 1 whose top ơi is in X and whose bottom ơi 1 is in Y. Recently Hall and Remmel 4 proved two formulas for the number PnS of ơ E Sn with s X Y -descents which generalized Liese s results in 1 . We define a new statistic statX y ơ on permutations ơ and define PX q to be the sum of qstatv v ơ over all ơ E Sn with s X Y -descents. We then show that there are natural q-analogues of the Hall-Remmel formulas for Pnfsy q . 1 Introduction Let Sn denote the set of permutations of the set n 1 2 . n . Given subsets X Y c N and a permutation a E Sn let DesX y a i ai ai 1 ai E X ai 1 E Y and desx y a DesX y a . If i E DesX y a then we call the pair ai ai 1 an X Y -descent. For example if X 2 3 5 Y 1 3 4 and a 54213 then DesX y a 1 3 and desX y a 2. For fixed n we define the polynomial P x y PXs T Department of Mathematics Harvard University Cambridge MA hall@ Department of Mathematics California Polytechnic State University San Luis Obispo CA 93407 jliese@ Department of Mathematics University of California San Diego La Jolla CA 92093 jrem-mel@ This work partially supported by NSF grant DMS 0654060 THE ELECTRONIC JOURNAL OF COMBINATORICS 16 2009 R111 1 Thus the coefficient PffiY is the number of ơ G Sn with exactly s X Y -descents. Hall and Remmel 4 gave direct combinatorial proofs of a pair of formulas for P x. First of all for any set A c N let An A n n and An Ac n n A- Then Hall and Remmel 4 proved the following theorem. Theorem . PnY Xn ý 1 - Xn r n P n 1 r x n x Wn x z r J X s_r r 0 ỵ xeXn and P Xn T i Xn . X n J n r Px x r 0 V r VlXnI s V xtXn where for any set A and any j 1 j n

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