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A058696 Number of ways to partition 2n into positive integers. 14
1, 2, 5, 11, 22, 42, 77, 135, 231, 385, 627, 1002, 1575, 2436, 3718, 5604, 8349, 12310, 17977, 26015, 37338, 53174, 75175, 105558, 147273, 204226, 281589, 386155, 526823, 715220, 966467, 1300156, 1741630, 2323520, 3087735, 4087968 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

A bisection of A000041, the other one is A058695.

Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). - Michael Somos, Feb 16 2014

a(n) = number of partitions of 3n-2 having n as a part, for n >= 1.  Also, a(n+1) is the number of partitions of 3n having n as a part, for n >= 1.  - Clark Kimberling, Mar 02 2014

REFERENCES

Roland Bacher, P De La Harpe, Conjugacy growth series of some infinitely generated groups. 2016. hal-01285685v2; https://hal.archives-ouvertes.fr/hal-01285685/document

LINKS

Seiichi Manyama, Table of n, a(n) for n = 0..10000

M. Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

FORMULA

Expansion of f(x^3, x^5) / f(-x)^2 in powers of x where f() is a Ramanujan theta function. - Michael Somos, Feb 16 2014

Euler transform of period 16 sequence [ 2, 2, 3, 2, 3, 1, 2, 1, 2, 1, 3, 2, 3, 2, 2, 1, ...]. - Michael Somos,  Apr 25 2003

a(n) = A000041(2*n).

Convolution of A000041 and A035294. - Michael Somos, Feb 16 2014

G.f.: Product_{k>=1} (1 + x^(8*k-4)) * (1 + x^(8*k)) * (1 + x^k)^2 / ((1 + x^(8*k-1)) * (1 + x^(8*k-7)) * (1 - x^k)). - Vaclav Kotesovec, Nov 17 2016

EXAMPLE

G.f. = 1 + 2*x + 5*x^2 + 11*x^3 + 22*x^4 + 42*x^5 + 77*x^6 + 135*x^7 + ...

G.f. = q^-1 + 2*q^47 + 5*q^95 + 11*q^143 + 22*q^191 + 42*q^239 + 77*q^287 + ...

MAPLE

with(combinat): with(numtheory): a := proc(n) c := 1: l := sort(convert(divisors(n), list)): for i from 1 to nops(l) do c := numbpart(l[i]*2) od: RETURN(c): end: for j from 0 to 61 do printf(`%d, `, a(j)) od: # Zerinvary Lajos, Apr 14 2007

MATHEMATICA

nn=100; Table[CoefficientList[Series[Product[1/(1-x^i), {i, 1, nn}], {x, 0, nn}], x][[2i-1]], {i, 1, nn/2}] (* Geoffrey Critzer, Sep 28 2013 *)

(* also *)

Table[PartitionsP[2 n], {n, 0, 40}] (* Clark Kimberling, Mar 02 2014 *)

(* also *)

Table[Count[IntegerPartitions[3 n - 2], p_ /; MemberQ[p, n]], {n, 20}]   (* Clark Kimberling, Mar 02 2014 *)

nmax = 60; CoefficientList[Series[Product[(1 + x^(8*k-4))*(1 + x^(8*k))*(1 + x^k)^2/((1 + x^(8*k-1))*(1 + x^(8*k-7))*(1 - x^k)), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Nov 17 2016 *)

PROG

(PARI) {a(n) = if( n<0, 0, polcoeff( 1 / eta(x + O(x^(2*n + 1))), 2*n))}; /* Michael Somos, Apr 25 2003 */

(PARI) a(n) = numbpart(2*n); \\ Michel Marcus, Sep 28 2013

(MuPAD) combinat::partitions::count(2*i) $i=0..54 // Zerinvary Lajos, Apr 16 2007

CROSSREFS

Cf. A000041, A035294, A058695.

Sequence in context: A290778 A291590 A236430 * A134508 A091357 A309950

Adjacent sequences:  A058693 A058694 A058695 * A058697 A058698 A058699

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Dec 31 2000

STATUS

approved

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Last modified October 14 17:43 EDT 2019. Contains 328022 sequences. (Running on oeis4.)