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 A117045 Integers n (not perfect squares) such that the continued fraction expansion of the square root of n has period at most 2. 0
 2, 3, 5, 6, 8, 10, 11, 15, 17, 18, 20, 24, 26, 27, 30, 35, 37, 38, 39, 40, 42, 48, 50, 51, 56, 63, 65, 66, 68, 72, 80, 82, 83, 84, 87, 90 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS In a recent paper Justin Thomas, myself and Julian Rosen show that this is equivalent to the following criterion: let d be the integer part of the square root. Then sqrt{n} has period at most 2 if and only if 2d/(n - d^2) is an integer. REFERENCES Thomas, J., Shankar, K., Rosen, J., "Continued Fractions, Square Roots and the orbit of 1/0 on the boundary of the hyperbolic plane", preprint. LINKS K. Shankar, Square roots and continued fractions. EXAMPLE The first term is 2 because sqrt{2} is irrational and for n=2, d=1, 2d/(n - d^2) = 1 is an integer. CROSSREFS Sequence in context: A179180 A085921 A005243 * A324793 A244053 A275833 Adjacent sequences:  A117042 A117043 A117044 * A117046 A117047 A117048 KEYWORD nonn AUTHOR Krishnan Shankar (shankar(AT)math.ou.edu), Apr 17 2006 STATUS approved

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Last modified October 15 20:04 EDT 2019. Contains 328037 sequences. (Running on oeis4.)