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)).
Empirical g.f.: (1-3*x+6*x^2-7*x^3+9*x^4-7*x^5+6*x^6-3*x^7+x^8) / ((1-x)^5*(1+x^2)*(1+x+x^2)).
PROG
(PARI)
\\ b(n) is the coefficient of x^n in the g.f. 1/((1-x)*(1-x^2)*(1-x^3)).
b(n) = round(real((47+9*(-1)^n + 8*exp(-2/3*I*n*Pi) + 8*exp((2*I*n*Pi)/3) + 36*n+6*n^2)/72))
vector(50, n, b(n*(n-1)/2))
CROSSREFS
KEYWORD
nonn
AUTHOR
Colin Barker, Jun 15 2016
STATUS
approved