OFFSET
1,3
COMMENTS
This is "one-fifteenth of triangular numbers (integers only)". - Vladimir Joseph Stephan Orlovsky, Mar 04 2009
LINKS
Harry J. Smith, Table of n, a(n) for n = 1..20000
Index entries for linear recurrences with constant coefficients, signature (1,0,0,2,-2,0,0,-1,1).
FORMULA
a(-1 - n) = a(n). a(n) = (A057538(n) * 2 - 1) / 120.
G.f. -x^2*(1+2*x+4*x^2+x^3+4*x^4+x^6+2*x^5) / ( (1+x)^2*(x^2+1)^2*(x-1)^3 ). - R. J. Mathar, Jun 09 2013
MATHEMATICA
lst={}; s=0; Do[s+=n/15; If[Floor[s]==s, AppendTo[lst, s]], {n, 0, 7!}]; lst (* Vladimir Joseph Stephan Orlovsky, Mar 04 2009 *)
Select[Table[(n^2-1)/120, {n, 0, 700}], IntegerQ] (* Harvey P. Dale, Nov 26 2010 *)
PROG
(PARI) {a(n) = (((n\4 * 3 + n%4) * 10 + (-1)^(n\2))^2 - 1) / 120 } /* Michael Somos, Oct 17 2006 */
(PARI) { for (n=0, 19999, a=(((n\4*3 + n%4)*10 + (-1)^(n\2))^2 - 1)/120; write("b093722.txt", n+1, " ", a); ) } \\ Harry J. Smith, Jun 18 2009
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Michael Somos, Apr 13 2004
EXTENSIONS
More terms from Harvey P. Dale, Nov 26 2010
Offset corrected to 1 by Ray Chandler, Jul 29 2019
STATUS
approved