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A031561
Numbers k such that the continued fraction for sqrt(k) has even period and if the last term of the periodic part is deleted the central term is 63.
0
3971, 4003, 4007, 4019, 4027, 4039, 4051, 4059, 4079, 4083, 4091, 4099, 4111, 4127, 4131, 4139, 4151, 4159, 4183, 4207, 4211, 4219, 4223, 15880, 15936, 15944, 15968, 15976, 16000, 16064, 16160, 16192, 16224, 16232, 16288, 16328, 16424, 16480, 16544
OFFSET
1,1
COMMENTS
The "central term" is the term appearing at 1/2 the length of the period of the continued fraction, not the term succeeding that term. For instance, the periodic part of the continued fraction of sqrt(4139) is {2, 1, 63, 1, 2, 128}. - Harvey P. Dale, Sep 07 2012
MATHEMATICA
ct63Q[n_]:=Module[{sqrt=Sqrt[n], cf, len}, cf=If[IntegerQ[sqrt], {1}, ContinuedFraction[sqrt][[2]]]; len=Length[cf]; EvenQ[len] && cf[[len/2]]==63]; Select[ Range[17000], ct63Q] (* Harvey P. Dale, Sep 07 2012 *)
CROSSREFS
Sequence in context: A343024 A343002 A230067 * A031741 A204501 A204494
KEYWORD
nonn
STATUS
approved