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A061256 Euler transform of sigma(n), cf. A000203. 15
1, 1, 4, 8, 21, 39, 92, 170, 360, 667, 1316, 2393, 4541, 8100, 14824, 26071, 46422, 80314, 139978, 238641, 408201, 686799, 1156062, 1920992, 3189144, 5238848, 8589850, 13963467, 22641585, 36447544, 58507590, 93334008, 148449417, 234829969, 370345918 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

This is also the number of ordered triples of permutations f, g, h in Symm(n) which all commute, divided by n!. This was conjectured by Franklin T. Adams-Watters, Jan 16 2006, and proved by J. R. Britnell in 2012.

According to a message on a blog page by "Allan" (see http://sbseminar.wordpress.com/2010/10/06/a-peculiar-numerical-coincidence/#comments) it appears that a(n) = number of conjugacy classes of commutative ordered pairs in Symm(n).

John McKay (email to N. J. A. Sloane, Apr 23 2013) observes that A061256 and A006908 coincide for a surprising number of terms, and asks for an explanation. - N. J. A. Sloane, May 19 2013

LINKS

T. D. Noe, Table of n, a(n) for n = 0..1000

J. R. Britnell, A formal identity involving commuting triples of permutations, arXiv:1203.5079 [math.CO], 2012.

J. R. Britnell, A formal identity involving commuting triples of permutations, Journal of Combinatorial Theory, Series A, Volume 120, Issue 4, May 2013, Preprint 2012. - N. J. A. Sloane, Jun 13 2012

E. Marberg, How to compute the Frobenius-Schur indicator of a unipotent character of a finite Coxeter system, arXiv preprint arXiv:1202.1311 [math.RT], 2012. -  N. J. A. Sloane, Jun 10 2012

Tad White, Counting Free Abelian Actions, arXiv:1304.2830 [math.CO], 2013.

N. J. A. Sloane, Transforms

FORMULA

a(n) = A072169(n) / n!.

G.f.: Product_{k=1..infinity} (1 - x^k)^(-sigma(k)). a(n)=1/n*Sum_{k=1..n} a(n-k)*b(k), n>1, a(0)=1, b(k)=Sum_{d|k} d*sigma(d), cf. A001001.

G.f.: exp( Sum_{n>=1} sigma(n)*x^n/(1-x^n)^2 /n ). [Paul D. Hanna, Mar 28 2009]

G.f.: exp( Sum_{n>=1} sigma_2(n)*x^n/(1-x^n)/n ). [Vladeta Jovovic, Mar 28 2009]

G.f.: prod(n>=1, E(x^n)^n ) where E(x) = prod(k>=1, 1-x^k). [Joerg Arndt, Apr 12 2013]

EXAMPLE

1 + x + 4*x^2 + 8*x^3 + 21*x^4 + 39*x^5 + 92*x^6 + 170*x^7 + 360*x^8 + ...

MATHEMATICA

nn = 30; b = Table[DivisorSigma[1, n], {n, nn}]; CoefficientList[Series[Product[1/(1 - x^m)^b[[m]], {m, nn}], {x, 0, nn}], x] (* T. D. Noe, Jun 18 2012 *)

nmax = 40; CoefficientList[Series[Product[1/QPochhammer[x^k]^k, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Nov 29 2015 *)

PROG

(PARI) N=66; x='x+O('x^N); gf=1/prod(j=1, N, eta(x^j)^j); Vec(gf) /* Joerg Arndt, May 03 2008 */

(PARI) {a(n)=if(n==0, 1, polcoeff(exp(sum(m=1, n, sigma(m)*x^m/(1-x^m+x*O(x^n))^2/m)), n))} /* Paul D. Hanna, Mar 28 2009 */

CROSSREFS

Cf. A000203, A001001, A006171, A001970, A053529, A061255, A061257, A006908.

Sequence in context: A233401 A006908 A079860 * A180608 A244583 A261031

Adjacent sequences:  A061253 A061254 A061255 * A061257 A061258 A061259

KEYWORD

easy,nonn

AUTHOR

Vladeta Jovovic, Apr 21 2001

EXTENSIONS

Entry revised by N. J. A. Sloane, Jun 13 2012

STATUS

approved

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Last modified May 28 07:57 EDT 2017. Contains 287212 sequences.