OFFSET
0,4
COMMENTS
a(n) is the number of palindromic word structures of length n using 10-ary alphabet.
a(n) is the same as taking every element twice from A164864.
LINKS
Index entries for linear recurrences with constant coefficients, signature (1, 45, -45, -861, 861, 9135, -9135, -58674, 58674, 233100, -233100, -557864, 557864, 732960, -732960, -403200, 403200).
FORMULA
G.f.: (148329*x^17 -403200*x^16 -210253*x^15 +732960*x^14 +122692*x^13 -557864*x^12 -38365*x^11 +233100*x^10 +6965*x^9 -58674*x^8 -736*x^7 +9135*x^6 +42*x^5 -861*x^4 -x^3 +45*x^2 -1) / ((x -1)*(2*x -1)*(2*x +1)*(2*x^2 -1)*(3*x^2 -1)*(5*x^2 -1)*(6*x^2 -1)*(7*x^2 -1)*(8*x^2 -1)*(10*x^2 -1)). [Colin Barker, Dec 05 2012]
EXAMPLE
Four-digit palindromes have two different digits structures: aaaa and abba. Hence a(4)=2.
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Tanya Khovanova, Aug 30 2009
STATUS
approved