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A031415 Numbers n such that continued fraction for sqrt(n) has odd period and a pair of central terms both equal to 2. 1
41, 61, 113, 130, 181, 202, 265, 269, 313, 317, 394, 421, 458, 586, 613, 617, 685, 697, 761, 773, 853, 925, 929, 937, 986, 1013, 1066, 1109, 1117, 1201, 1213, 1301, 1325, 1354, 1409, 1417, 1429, 1466, 1586, 1625, 1637, 1649, 1714, 1741, 1745, 1753, 1861 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

In general, the simple continued fraction expansion of sqrt(m) is a periodic palindromic sequence. That is, contfrac( sqrt(m) ) = [c(0); c(1), c(2), ..., c(p), c(p+1), ...] where p is the period. c(p) = 2*c(0), c(k) = c(p+k) for k>0, c(k) = c(p-k) for p>k>0. If the period p is odd, then p = 2*k+1 and c(k) = c(k+1) can be considered a pair of equal central terms. If the period is even, then p = 2*k and the unique central term is c(k). - Michael Somos, Apr 04 2014

LINKS

T. D. Noe, Table of n, a(n) for n = 1..999

EXAMPLE

The simple continued fraction expansion of sqrt(41) = [6; 2, 2, 12,  2, 2, 12, 2, 2, 12, ...] with odd period 3 and two terms equal to 2. Another example is sqrt(202) = [14; 4, 1, 2, 2, 1, 4, 28, 4, 1, 2, 2, 1, 4, 28, 4, 1, 2, 2, 1, 4, 28,  ...] with odd period 7 and two terms equal to 2. - Michael Somos, Apr 03 2014

MATHEMATICA

n = 1; t = {}; While[Length[t] < 50, n++; If[! IntegerQ[Sqrt[n]], c = ContinuedFraction[Sqrt[n]]; len = Length[c[[2]]]; If[OddQ[len] && c[[2, (len + 1)/2]] == 2, AppendTo[t, n]]]]; t (* T. D. Noe, Apr 03 2014 *)

CROSSREFS

Cf. A031404-A031423.

Sequence in context: A139952 A195573 A260554 * A325072 A089345 A320468

Adjacent sequences:  A031412 A031413 A031414 * A031416 A031417 A031418

KEYWORD

nonn

AUTHOR

David W. Wilson

EXTENSIONS

a(1) corrected by T. D. Noe, Apr 03 2014

STATUS

approved

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Last modified September 21 08:38 EDT 2020. Contains 337268 sequences. (Running on oeis4.)