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 A128085 Central coefficients of q in the q-analog of the even double factorials: a(n) = [q^([n^2/2])] Product_{j=1..n} (1-q^(2j))/(1-q). 3
 1, 1, 2, 8, 46, 340, 3210, 36336, 484636, 7394458, 127707302, 2454109404, 52091631896, 1207854671388, 30431260261770, 826657521349952, 24114046688034516, 751085176539860458, 24899882719111953556 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS See A128081 for central coefficients of q in the q-analog of the odd double factorials. Also, A000140 is the central coefficients of q-factorials, giving the maximum number of permutations on n letters having the same number of inversions. LINKS Eric Weisstein's World of Mathematics, q-Factorial from MathWorld. EXAMPLE a(n) is the central term of the q-analog of even double factorials, in which the coefficients of q (triangle A128084) begin: n=0: (1); n=1: (1),1; n=2: 1,2,(2),2,1; n=3: 1,3,5,7,(8),8,7,5,3,1; n=4: 1,4,9,16,24,32,39,44,(46),44,39,32,24,16,9,4,1; n=5: 1,5,14,30,54,86,125,169,215,259,297,325,(340),340,325,297,...;... The terms enclosed in parenthesis are initial terms of this sequence. PROG (PARI) a(n)=if(n==0, 1, polcoeff(prod(k=1, n, (1-q^(2*k))/(1-q)), n^2\2, q)) CROSSREFS Cf. A000165 ((2n)!!); A128084 (triangle), A128086 (diagonal); A128081. Sequence in context: A111552 A321965 A229559 * A052801 A294784 A180390 Adjacent sequences:  A128082 A128083 A128084 * A128086 A128087 A128088 KEYWORD nonn AUTHOR Paul D. Hanna, Feb 14 2007 STATUS approved

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Last modified June 24 08:30 EDT 2021. Contains 345416 sequences. (Running on oeis4.)