A249349 (A001147(n+1)-1)/2, equals the index of A249348(n) within the triangular numbers A000217.

0, 1, 7, 52, 472, 5197, 67567, 1013512, 17229712, 327364537, 6874655287, 158117071612, 3952926790312, 106729023338437, 3095141676814687, 95949391981255312, 3166329935381425312, 110821547738349885937, 4100397266318945779687, 159915493386438885407812

Offset: 0

Author

M. F. Hasler, Oct 26 2014

Keywords

• nonn

Comments

Also a(n) = floor(sqrt(A249348(n)*2)).

The positive terms are of the form 3k-2; this k (= 1, 3, 18, 157, ...) is the index of A249348(n) within the centered 9-gonal numbers A060544.

Programs

PARI

a(n)=A001147(n+1)\2

PARI

vector(10,n,A001147(n)\2) \\ To get the initial term a(0) for n=1.

Formula

a(n) +(-2*n-3)*a(n-1) +(4*n-1)*a(n-2) +(-2*n+3)*a(n-3)=0. - R. J. Mathar, Oct 28 2014