A317975 a(n) = 2*(a(n-1)+a(n-2)+a(n-3))-a(n-4) for n >= 4, with initial terms 0, 1, 1, 0.

0, 1, 1, 0, 4, 9, 25, 76, 216, 625, 1809, 5224, 15100, 43641, 126121, 364500, 1053424, 3044449, 8798625, 25428496, 73489716, 212389225, 613816249, 1773961884, 5126845000, 14816857041, 42821511601, 123756465400, 357662823084, 1033664743129, 2987346551625

Offset: 0

Author

N. J. A. Sloane, Sep 03 2018

Keywords

• nonn
• easy

Links

• Andrew Howroyd, Table of n, a(n) for n = 0..1000
• H. S. M. Coxeter, Loxodromic sequences of tangent spheres, Aequationes Mathematicae, 1.1-2 (1968): 104-121. See p. 112.
• Index entries for linear recurrences with constant coefficients, signature (2,2,2,-1)

Programs

Mathematica

LinearRecurrence[{2, 2, 2, -1}, {0, 1, 1, 0}, 31] (* Jean-François Alcover, Sep 13 2018 *)

PARI

concat([0], Vec((1 - x - 4*x^2)/(1 - 2*x - 2*x^2 - 2*x^3 + x^4) + O(x^40))) \\ Andrew Howroyd, Sep 08 2018

Formula

G.f.: x*(1 - x - 4*x^2)/(1 - 2*x - 2*x^2 - 2*x^3 + x^4). - Andrew Howroyd, Sep 08 2018

Extensions

Terms a(10) and beyond from Andrew Howroyd, Sep 08 2018