A238219 The total number of 4's in all partitions of n into an even number of distinct parts.

0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 2, 1, 2, 3, 4, 4, 5, 6, 8, 9, 11, 13, 16, 18, 21, 25, 29, 34, 40, 46, 53, 62, 71, 82, 94, 108, 124, 142, 161, 185, 210, 238, 270, 307, 347, 392, 442, 499, 562, 632, 709, 797, 894, 1000, 1119, 1252, 1398, 1560, 1739, 1937, 2157

Offset: 0

Author

Mircea Merca, Feb 20 2014

Keywords

• nonn

Comments

The g.f. for "number of k's" is (1/2)*(x^k/(1+x^k))*(Product_{n>=1} 1 + x^n) - (1/2)*(x^k/(1-x^k))*(Product_{n>=1} 1 - x^n).

Examples

			a(13) = 3 because the partitions in question are: 9+4, 6+4+2+1, 5+4+3+1.
		

Links

• Andrew Howroyd, Table of n, a(n) for n = 0..1000

Crossrefs

Column k=4 of A238451.

Cf. A067659, A067661.

Programs

Mathematica

nmax = 100; With[{k=4}, CoefficientList[Series[x^k/(1+x^k)/2 * Product[1 + x^j, {j, 1, nmax}] - x^k/(1-x^k)/2 * Product[1 - x^j, {j, 1, nmax}], {x, 0, nmax}], x]] (* Vaclav Kotesovec, Jul 05 2025 *)

Formula

a(n) = Sum_{j=1..round(n/8)} A067659(n-(2*j-1)*4) - Sum_{j=1..floor(n/8)} A067661(n-8*j).

G.f.: (1/2)*(x^4/(1+x^4))*(Product_{n>=1} 1 + x^n) - (1/2)*(x^4/(1-x^4))*(Product_{n>=1} 1 - x^n).

a(n) ~ exp(Pi*sqrt(n/3)) / (16 * 3^(1/4) * n^(3/4)). - Vaclav Kotesovec, Jul 05 2025

Extensions

Terms a(51) and beyond from Andrew Howroyd, Apr 29 2020