OFFSET
0,2
COMMENTS
A059100 has a totally different description but is the same sequence (omitting the first two numbers here)
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..1000
Daniel Bach ("Dan"), Kissing circles.
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = a(1) + a(2) + a(n-1) + 2*sqrt(a(1)*a(2) + a(1)*a(n-1) + a(2)*a(n-1)) = 1 + a(n-1) + 2*(sqrt(-2 + a(n-1))). (Descartes' curvature-theorem)
From Colin Barker, Jan 07 2013: (Start)
a(n) = n^2 - 4*n + 6 for n > 1.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 4.
G.f.: -(x^4 + 4*x^3 - 7*x^2 + 5*x - 1)/(x - 1)^3.
(End)
PROG
(PARI) a(n) = if(n>1, n^2 - 4*n + 6, [-1, 2][n+1]) \\ Andrew Howroyd, Feb 25 2018
CROSSREFS
KEYWORD
sign,easy
AUTHOR
Bodo Zinser, Feb 10 2005
STATUS
approved