OFFSET
0,3
COMMENTS
a(n) is the number of free polyominoes of width 2 and height n+1 which have no symmetry, i.e., rotations by 180 degrees, flips along the short or long axis generate a different free polyomino. The three elements t, g+ and g- of sequences by Tasi et al. represent a domino in the short cross-section where either both, only the "upper" or only the "lower" square of the domino is occupied. E.g., a(3) = 8 represents 3 5-ominoes of shape the 2x4, 3 6-ominoes of shape 2x4, and 2 7-ominoes of shape 2x4. - R. J. Mathar, Jun 17 2020
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Gy. Tasi and F. Mizukami, Quantum algebraic-combinatoric study of the conformational properties of n-alkanes, J. Math. Chemistry, 25, 1999, 55-64 (see p. 63, eq 25).
Index entries for linear recurrences with constant coefficients, signature (3,1,-7,3,-1,1,1).
FORMULA
4*a(n) = Pell(n+3) - Pell(n+2) - 2*Pell(floor((n+4)/2)) + 1, with Pell(n) = A000129(n). - Ralf Stephan, Jun 02 2007
G.f.: x*(1-x+x^2)/((1-x)*(1-2*x-x^2)*(1-2*x^2-x^4)). - Colin Barker, Apr 08 2013
From G. C. Greubel, May 24 2021: (Start)
MATHEMATICA
Table[(LucasL[n+2, 2] -4*Fibonacci[Floor[n/2]+2, 2] +2)/8, {n, 0, 40}] (* G. C. Greubel, May 24 2021 *)
PROG
(Sage) [(lucas_number2(n+2, 2, -1) -4*lucas_number1(2+(n//2), 2, -1) +2)/8 for n in (0..40)] # G. C. Greubel, May 24 2021
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch, Jun 12 2005
EXTENSIONS
Entry revised by N. J. A. Sloane, Jul 29 2011
STATUS
approved