OFFSET
1,2
COMMENTS
The terms are exactly the odd pentagonal numbers; that is, they are all the odd numbers of the form k*(3*k-1)/2 where k is an integer. - James A. Sellers, Jun 09 2007
Apparently groups of two odd pentagonal numbers (A000326, A014632) followed by two odd 2nd pentagonal numbers (A005449), which leads to the conjectured generating function x*(x^2+4*x+1)*(x^4-2*x^3+4*x^2-2*x+1)/((x^2+1)^2*(1-x)^3). - R. J. Mathar, Jul 26 2009
Odd generalized pentagonal numbers. - Omar E. Pol, Aug 19 2011
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
FORMULA
Sum_{n>=1} 1/a(n) = Pi/2. - Amiram Eldar, Aug 18 2022
MATHEMATICA
With[{nn=50}, Sort[Select[Table[(n(3n-1))/2, {n, -nn, nn}], OddQ]]] (* Harvey P. Dale, Feb 16 2014 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Naohiro Nomoto, Jan 31 2002
EXTENSIONS
Corrected by T. D. Noe, Oct 25 2006
STATUS
approved