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A127728 Sum of squared coefficients of q in the q-factorials. 5
1, 1, 2, 10, 106, 1930, 53612, 2108560, 111482424, 7625997280, 655331699940, 69110082376388, 8775534280695310, 1320693932817784342, 232459627389638257316, 47311901973588298051380 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Two n-permutations are randomly selected from S_n with replacement. a(n)/(n!)^2 is the probability that they will have the same number of inversions. - Geoffrey Critzer, May 15 2010

LINKS

Table of n, a(n) for n=0..15.

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

EXAMPLE

Definition of q-factorial of n:

faq(n) = Product_{k=1..n} (1-q^k)/(1-q) for n>0, with faq(0)=1;

faq(4) = 1*(1 + q)*(1 + q + q^2)*(1 + q + q^2 + q^3) = 1 + 3*q + 5*q^2 + 6*q^3 + 5*q^4 + 3*q^5 + q^6;

then a(n) is the sum of squared coefficients of q:

a(4) = 1^2 + 3^2 + 5^2 + 6^2 + 5^2 + 3^2 + 1^2 = 106.

MATHEMATICA

Table[Total[ CoefficientList[Expand[Product[Sum[x^i, {i, 0, m}], {m, 1, n - 1}]], x]^2], {n, 0, 15}] (* Geoffrey Critzer, May 15 2010 *)

PROG

(PARI) {a(n)=local(faq_n=if(n==0, 1, prod(k=1, n, (1-q^k)/(1-q)))); sum(k=0, n*(n-1)/2, polcoeff(faq_n, k, q)^2)}

CROSSREFS

Sequence in context: A273961 A049538 A217901 * A306064 A185396 A003222

Adjacent sequences:  A127725 A127726 A127727 * A127729 A127730 A127731

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Jan 25 2007

STATUS

approved

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Last modified June 6 04:15 EDT 2020. Contains 334859 sequences. (Running on oeis4.)