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%I #21 Jan 22 2018 16:00:57
%S 1,1,1,5,1,1,5,1,3,1,5,7,11,1,7,5,1,5,5,11,1,9,15,9,1,5,3,9,1,9,17,1,
%T 5,21,5,13,1,7,5,1,5,11,17,7,1,9,3,7,21,13,1,5,11,17,7,11,1,19,5,5,7,
%U 15,1,5,3,5,9,21,21,1,21,37,7,21,1,5,17,25,3,15,13,1,9,19,19,1,5,7,39
%N Length of period of continued for sqrt(A003814(n)).
%C If the period length of the continued fraction of a quadratic surd sqrt(D) is odd, the negative Pell equation x^2 - D y^2 = -1 has (infinite) solutions.
%H Ray Chandler, <a href="/A010333/b010333.txt">Table of n, a(n) for n = 1..10000</a>
%t Select[Map[Length@ Last@ ContinuedFraction@ Sqrt@ # &, Range[550]], OddQ] (* _Michael De Vlieger_, Dec 06 2017, after _T. D. Noe_ at A003814 *)
%Y Cf. A003814.
%K nonn
%O 1,4
%A _N. J. A. Sloane_, Walter Gilbert
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