A128085 - OEIS

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).

1, 1, 2, 8, 46, 340, 3210, 36336, 484636, 7394458, 127707302, 2454109404, 52091631896, 1207854671388, 30431260261770, 826657521349952, 24114046688034516, 751085176539860458, 24899882719111953556

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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

Table of n, a(n) for n=0..18.

Eric Weisstein's World of Mathematics, q-Factorial.

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,...;...