OFFSET
1,3
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
FORMULA
Coefficient of x^(n*(n-1)/2) in 1/((1-x)*(1-x^2)*(1-x^3)*(1-x^4)).
Empirical g.f.: (1 -5*x +15*x^2 -30*x^3 +54*x^4 -77*x^5 +109*x^6 -128*x^7 +150*x^8 -148*x^9 +150*x^10 -128*x^11 +109*x^12 -77*x^13 +54*x^14 -30*x^15 +15*x^16 -5*x^17 +x^18) / ((1 -x)^7*(1 +x^2)^3*(1 +x +x^2)*(1 +x^4)). - Colin Barker, Jun 12 2016
MATHEMATICA
Length[IntegerPartitions[#, 4]]&/@Accumulate[Range[0, 40]] (* Harvey P. Dale, Jul 08 2022 *)
PROG
(PARI)
\\ b(n) is the coefficient of x^n in the g.f. 1/((1-x)*(1-x^2)*(1-x^3)*(1-x^4)).
b(n) = round(real((68+36*(-1)^n+18*((-I)^n+I^n)+(16*exp(-2/3*I*n*Pi)*(1+I*sqrt(3)+2*exp((4*I*n*Pi)/3)))/(1+(-1)^(1/3))+59*(1+n)+9*(-1)^n*(1+n)+18*(1+n)*(2+n)+2*(1+n)*(2+n)*(3+n))/288))
vector(50, n, b(n*(n-1)/2)) \\ Colin Barker, Jun 12 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 11 2016
EXTENSIONS
More terms from Colin Barker, Jun 12 2016
STATUS
approved