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 A091809 Given the infinite continued fraction i+(i/(i+(i/(i+...)))), where i is the square root of (-1), this is the denominator of the imaginary part of the convergents. 7
 1, 1, 2, 5, 3, 10, 41, 85, 178, 123, 769, 10, 3329, 533, 1602, 30005, 62441, 64970, 270409, 187575, 1171042, 2436961, 5071361, 16490, 1045821, 45703841, 95110562, 15225145, 411889609, 47619450, 1783745641, 3712008565, 7724760338 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The sequence of complex numbers (which this sequence is part of) converges to (i+sqrt(-1+4i))/2, found by simply solving the equation A=i+(i/A) for A using the quadratic formula. When plotted in the complex plane, these numbers form a counterclockwise spiral that quickly converges to a point. LINKS EXAMPLE a(6) = 10 since the sixth convergent is (3/5) + (13/10)i and hence the denominator of the imaginary part is 10. MATHEMATICA GenerateA091809 := I; GenerateA091809[n_] := I + I/(GenerateA091809[n-1]); GenerateDenominatorsA091809[n_] := Table[Denominator[Im[GenerateA091809[x]]], {x, 1, n}]; GenerateDenominatorsA091809 gives the first 20 terms. A091809[n_] := Denominator[ IM[ Fold[ I/(I + #) &, 1, Range[n]]]]; Table[ A091809[n], {n, 0, 32}] (* Robert G. Wilson v, Mar 13 2004 *) CROSSREFS Cf. A091806, A091807, A091808. Sequence in context: A097753 A120860 A259971 * A110315 A275677 A221183 Adjacent sequences:  A091806 A091807 A091808 * A091810 A091811 A091812 KEYWORD cofr,frac,nonn AUTHOR Ryan Witko (witko(AT)nyu.edu), Mar 06 2004 STATUS approved

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Last modified October 25 13:59 EDT 2021. Contains 348253 sequences. (Running on oeis4.)