OFFSET
0,2
COMMENTS
Unit together with numbers of form (6*n+5)^2.
Sequence may be obtained by starting with the segment (1, 25) followed by the line from 25 in the direction 25, 121,... in the square spiral whose vertices are the generalized 20-gonal numbers. - Omar E. Pol, Jul 29 2016
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = A016970(n-1), n >= 1.
G.f.: (49*x^2 + 22*x + 1)/(1 - x)^3. - Vincenzo Librandi, Jan 27 2013
a(n) = 6*A033579(n) + 1. - Miquel Cerda, Jul 28 2016
a(n) = 36n^2 - 12n + 1. - Omar E. Pol, Jul 28 2016
E.g.f.: exp(x)*(1 + 24*x + 36*x^2). - Stefano Spezia, Aug 19 2023
EXAMPLE
a(0)=1 because (6*0-1)^2=1, a(1)=25 because (6*1-1)^2=25.
MATHEMATICA
CoefficientList[Series[(49*x^2 + 22*x + 1)/(1 - x)^3, {x, 0, 40}], x] (* Vincenzo Librandi, Jan 27 2013 *)
PROG
(Magma) [(6*n-1)^2: n in [0..50]]; // Vincenzo Librandi, May 07 2011
(PARI) a(n)=(6*n-1)^2 \\ Charles R Greathouse IV, Jul 28 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Juri-Stepan Gerasimov, Mar 17 2010
EXTENSIONS
Offset and formula corrected by R. J. Mathar, Apr 16 2010
STATUS
approved