OFFSET
1,2
COMMENTS
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (9,-9,1).
FORMULA
a(n) = 9*a(n-1)-9*a(n-2)+a(n-3).
G.f.: x*(4*x-1) / ((x-1)*(x^2-8*x+1)).
a(n) = (6-(4-sqrt(15))^n*(3+sqrt(15))+(-3+sqrt(15))*(4+sqrt(15))^n)/12. - Colin Barker, Mar 03 2016
EXAMPLE
5 is in the sequence because the 5th centered triangular number is 31, which is also the 4th centered pentagonal number.
PROG
(PARI) Vec(x*(4*x-1)/((x-1)*(x^2-8*x+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jan 01 2015
STATUS
approved