OFFSET
1,1
COMMENTS
Always a square.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..300
Tanya Khovanova, Recursive Sequences
Index entries for linear recurrences with constant coefficients, signature (9).
FORMULA
a(n) = 121*3^(2n-2).
EXAMPLE
a(2) = 3^2 + 3^3 + 3^4 + 3^5 + 3^6 = 121*(3^2) = 1089.
MATHEMATICA
Total/@Select[Partition[3^Range[0, 60], 5, 1], IntegerQ[Log[9, First[#]]]&] (* or *) Table[121 3^(2n-2), {n, 30}] (* Harvey P. Dale, May 03 2011 *)
PROG
(Magma) [121*3^(2*n-2): n in [1..30]]; // Vincenzo Librandi, Jun 10 2011
(PARI) a(n)=121*9^(n-1) \\ Charles R Greathouse IV, Jul 11 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Anton Joha, Jun 25 2006
EXTENSIONS
Corrected and extended by Harvey P. Dale, May 03 2011
STATUS
approved