OFFSET
1,1
COMMENTS
Conjecture: For n>=2, a(n) is the difference between the greater of successive pairs of odd triangular numbers(A000217), namely A(B(n-1)+1) - A(B(n-2)+1), where A=A000217 and B=A016813. This reduces to a(n)=2*(8*n-7) (see Formula). (1,3),(15,21) are the first consecutive pairs of odd triangular numbers, and 21-3=18, which is a(2). - David James Sycamore, Sep 12 2019
LINKS
William A. Stein, Dimensions of the spaces S_k(Gamma_0(N)).
William A. Stein, The modular forms database
FORMULA
Conjectures from Colin Barker, Jun 09 2019: (Start)
G.f.: x*(3 + 12*x + x^2) / (1 - x)^2.
a(n) = 2*a(n-1) - a(n-2) for n>3.
a(n) = 2*(8*n-7) for n>1.
(End)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 08 2001
STATUS
approved