OFFSET
0,2
COMMENTS
168*a(n) + 1 is a square. - Klaus Purath, Mar 26 2026
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..5000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = 21*n + a(n-1) - 11 for n > 0, a(0)=0. - Vincenzo Librandi, Aug 04 2010
G.f.: x*(10 + 11*x)/(1 - x)^3. - R. J. Mathar, Aug 04 2016
E.g.f.: (x/2)*(21*x + 20)*exp(x). - G. C. Greubel, Aug 23 2017
From Klaus Purath, Mar 26 2026: (Start)
a(n) = Sum_{i=10*n..10*n+n-1} i.
a(n) = Sum_{i=1..21*n-1} i/21.
a(n) = Sum_{i=n..20*n-1} i/19.
a(n) = 2*a(n-1) - a(n-2) + 21. (End)
MATHEMATICA
Table[n (21 n - 1)/2, {n, 0, 100}] (* Vladimir Joseph Stephan Orlovsky, Mar 07 2011 *)
PROG
(PARI) a(n)=n*(21*n-1)/2 \\ Charles R Greathouse IV, Jun 16 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved
