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 A028231 From hexagons in a circle problem. 2
 1, 22, 313, 4366, 60817, 847078, 11798281, 164328862, 2288805793, 31878952246, 444016525657, 6184352406958, 86136917171761, 1199732487997702, 16710117914796073, 232741918319147326, 3241676738553266497, 45150732421426583638, 628868577161418904441 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Numbers k such that (k^2 + k + 1)/3 is a square. - Arkadiusz Wesolowski, Feb 10 2012 Given by the numerators of the convergents to the continued fraction [1,(1,2)^i,3,(1,2)^{i-1},1]. - Jeffrey Shallit, Dec 11 2017 REFERENCES J. D. E. Konhauser et al., Which Way Did the Bicycle Go?, MAA 1996, p. 104. T. Nagell, Des équations indéterminées  x^2 + x + 1 = y^n  et  x^2 + x + 1 = 3*y^n, Norsk Mat. Forenings Skrifter, Ser. I, (1921). LINKS Michel Marcus, Table of n, a(n) for n = 0..100 Kevin A. Broughan, An explicit bound for aliquot cycles of repdigits, #A15 INTEGERS  Vol 12  (2012) p. 4. Index entries for linear recurrences with constant coefficients, signature (15,-15,1) FORMULA a(n) = sqrt(3)*((2+sqrt(3))^(2*n+1) - (2-sqrt(3))^(2*n+1))/4 - 1/2 (see Kevin A. Broughan paper). - Michel Marcus, Jul 28 2012 a(n) = 15*a(n-1)-15*a(n-2)+a(n-3). G.f.: (1+7*x-2*x^2)/((1-x)*(1-14*x+x^2)). - conjectured by Colin Barker, Apr 10 2012; these follow easily from the formula. MAPLE f:= gfun:-rectoproc({a(n) = 15*a(n-1)-15*a(n-2)+a(n-3), a(0)=1, a(1)=22, a(2)=313}, a(n), remember): map(f, [\$0..30]); # Robert Israel, Dec 12 2017 MATHEMATICA With[{k = Sqrt@ 3}, Simplify@ Array[k ((2 + k)^(2 # + 1) - (2 - k)^(2 # + 1))/4 - 1/2 &, 19, 0]] (* Michael De Vlieger, Dec 11 2017 *) PROG (PARI) a(n) = {w = quadgen(12); w*((2+w)^(2*n+1) - (2-w)^(2*n+1))/4 - 1/2; } /* Michel Marcus, Jul 28 2012 */ CROSSREFS Cf. A001570, which gives the corresponding values of y in 3y^2 = n^2 + n + 1. - Jeffrey Shallit, Dec 11 2017 Sequence in context: A028038 A025992 A028034 * A025988 A267132 A023949 Adjacent sequences:  A028228 A028229 A028230 * A028232 A028233 A028234 KEYWORD nonn,easy AUTHOR EXTENSIONS Edited by Robert Israel, Dec 12 2017 STATUS approved

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Last modified August 18 22:07 EDT 2018. Contains 313840 sequences. (Running on oeis4.)