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A225367 Number of palindromes of length n in base 3 (A118594). 5
3, 2, 6, 6, 18, 18, 54, 54, 162, 162, 486, 486, 1458, 1458, 4374, 4374, 13122, 13122, 39366, 39366, 118098, 118098, 354294, 354294, 1062882, 1062882, 3188646, 3188646, 9565938, 9565938, 28697814, 28697814, 86093442, 86093442, 258280326, 258280326, 774840978 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also: The number of n-digit terms in A006072. See there for further comments.

A palindrome of length L=2k-1 or of length L=2k is determined by the first k digits, which then determine the last k digits by symmetry. Since the first digit cannot be 0 (unless L=1), there are 2*3^(k-1) possibilities for L>1.

Except for the initial term, this is identical to A117855, which counts only nonzero palindromes.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (0,3).

FORMULA

a(n) = 2*3^floor((n-1)/2) + [n=1].

a(n) = 3*a(n-2) for n>3.

G.f.: x*(3*x^2-2*x-3)/(3*x^2-1).

a(n) = (6-(1+(-1)^n)*(3-sqrt(3)))*sqrt(3)^(n-3) for n>1, a(1)=3. [Bruno Berselli, May 06 2013]

EXAMPLE

The a(1)=3 palindromes of length 1 are: 0, 1 and 2.

The a(2)=2 palindromes of length 2 are: 11 and 22.

MATHEMATICA

Join[{3}, LinearRecurrence[{0, 3}, {2, 6}, 40]] (* Vincenzo Librandi, May 31 2017 *)

PROG

(PARI) A225367(n)=2*3^((n-1)\2)+!n

(MAGMA) [n eq 1 select 3 else 2*3^Floor((n-1)/2): n in [1..40]]; // Bruno Berselli, May 06 2013

(MAGMA) I:=[3, 2, 6]; [n le 3 select I[n] else 3*Self(n-2): n in [1..40]]; // Vincenzo Librandi, May 31 2017

CROSSREFS

Cf. A050683 and A070252 for base 10 analogs.

Sequence in context: A014686 A053090 A264400 * A283479 A087237 A275630

Adjacent sequences:  A225364 A225365 A225366 * A225368 A225369 A225370

KEYWORD

nonn,base,easy

AUTHOR

M. F. Hasler, May 05 2013

STATUS

approved

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Last modified June 25 08:21 EDT 2019. Contains 324347 sequences. (Running on oeis4.)