A117857 Number of palindromes of length n (in base 5).

4, 4, 20, 20, 100, 100, 500, 500, 2500, 2500, 12500, 12500, 62500, 62500, 312500, 312500, 1562500, 1562500, 7812500, 7812500, 39062500, 39062500, 195312500, 195312500, 976562500, 976562500, 4882812500, 4882812500, 24414062500, 24414062500, 122070312500

Offset

1,1

Links

Table of n, a(n) for n=1..31.

Index entries for linear recurrences with constant coefficients, signature (0,5).

Formula

a(n) = 4*5^floor((n-1)/2).

a(n) = 5*a(n-2). G.f.: -4*x*(x+1)/(5*x^2-1). [Colin Barker, Feb 15 2013]

Maple

A117857:=n->4*5^floor((n-1)/2): seq(A117857(n), n=1..40); # Wesley Ivan Hurt, Apr 18 2017

Mathematica

LinearRecurrence[{0, 5}, {4, 4}, 40] (* or *) With[{c=NestList[ 5#&, 4, 20]}, Riffle[ c, c]] (* Harvey P. Dale, Apr 18 2019 *)

Crossrefs

Cf. A050683.

Sequence in context: A035413 A261568 A087213 * A165559 A180967 A231884

Adjacent sequences: A117854 A117855 A117856 * A117858 A117859 A117860

Keyword

nonn,base,easy

Author

Martin Renner, May 02 2006

Extensions

More terms from Colin Barker, Feb 15 2013

Status

approved