OFFSET
1,4
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
K. Bezdek, P. Brass and H. Harborth, Maximum convex hulls of connected systems of segments and of polyominoes, Beiträge Algebra Geom., Vol. 35(1) (1994), pp. 37-43.
S. Kurz, Polyominoes with maximum convex hull, Diploma thesis, Bayreuth (2004).
Index entries for linear recurrences with constant coefficients, signature (0,2,0,1,0,-4,0,1,0,2,0,-1).
FORMULA
a(n) = (n^3 - 2*n^2 + 4*n)/16 if n mod 4 = 0;
a(n) = (n^3 - 2*n^2 + 13*n + 20)/32 if n mod 4 = 1;
a(n) = (n^3 - 2*n^2 + 4*n + 8)/16 if n mod 4 = 2;
a(n) = (n^3 - 2*n^2 + 5*n + 8)/32 if n mod 4 = 3.
G.f.: (1 + x - x^2 - x^3 + 2*x^5 + 8*x^6 + 2*x^7 + 4*x^8 + 2*x^9 - x^10 + x^12)/((1-x^2)^2*(1-x^4)^2).
From Luce ETIENNE, Aug 14 2019: (Start)
a(n) = 4*a(n-4) - 6*a(n-8) + 4*a(n-12) - a(n-16).
a(n) = 2*a(n-2) + a(n-4) - 4*a(n-6) + a(n-8) + 2*a(n-10) - a(n-12).
a(n) = (3*n^3 - 6*n^2 + 17*n + 22 + (n^3 - 2*n^2 - n - 6)*(-1)^n - 4*(4*cos(n*Pi/2) - (2*n+3)*sin(n*Pi/2)))/64. (End)
E.g.f.: (1/64)*(-exp(-x)*(6 - 2*x - x^2 + x^3) + exp(x)*(22 + 14*x + 3*x^2 + 3*x^3) - 4*(4*cos(x) - 2*x*cos(x) - 3*sin(x))). - Stefano Spezia, Aug 14 2019
MAPLE
A122133 := proc(n)
if modp(n, 4)= 0 then
(n^3-2*n^2+4*n)/16 ;
elif modp(n, 4)= 1 then
(n^3-2*n^2+13*n+20)/32 ;
elif modp(n, 4)= 2 then
(n^3-2*n^2+4*n+8)/16 ;
else
(n^3-2*n^2+5*n+8)/32 ;
fi;
end proc: # R. J. Mathar, May 19 2019
PROG
(PARI) Vec(x*(1+x-x^2+x^3+2*x^4+4*x^5+2*x^6+5*x^7+2*x^8+x^9)/((1-x)^4*(1+x)^4*(1+x^2)^2) + O(x^80)) \\ Colin Barker, Oct 14 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Sascha Kurz, Aug 21 2006
STATUS
approved