OFFSET
0,2
COMMENTS
Primes occur at indices n = 4, 66, 100, 102, 228, 346, ..., see A138647. - R. J. Mathar, Jan 19 2011
Sum_{i=0..m} (-1)^(m+i)*5^i, for m >= 0, gives all terms of the sequence. - Bruno Berselli, Aug 28 2013
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (4,5).
FORMULA
a(n) = (5*5^n + (-1)^n)/6.
G.f.: 1/((1+x)*(1-5x)).
E.g.f.: (5*exp(5x) + exp(-x))/6.
a(n) = Sum_{k=0..n} C(n-k,k)*4^(n-2k)*5^k. - Paul Barry, Jul 29 2004
a(n) = A015531(n+1). - R. J. Mathar, Sep 17 2008
a(n) = 4*a(n-1) + 5*a(n-2). - Vincenzo Librandi, Jun 23 2012
MAPLE
seq(coeff(series(factorial(n)*(5*exp(5*x)+exp(-x))/6, x, n+1), x, n), n = 0 .. 25); # Muniru A Asiru, Sep 21 2018
MATHEMATICA
LinearRecurrence[{4, 5}, {1, 4}, 40] (* Vincenzo Librandi, Jun 23 2012 *)
PROG
(Magma) [n le 2 select n^2 else 4*Self(n-1)+5*Self(n-2): n in [1..30]]; // Vincenzo Librandi, Jun 23 2012
(PARI) a(n)=(5*5^n+(-1)^n)/6 \\ Charles R Greathouse IV, Oct 07 2015
(GAP) List([0..25], n->(5*5^n+(-1)^n)/6); # Muniru A Asiru, Sep 21 2018
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Paul Barry, Apr 30 2003
STATUS
approved