A228934 - OEIS

%I #34 Dec 27 2016 02:36:00

%S 2,4,15,-99,-199,-800,-79201,-316808,-12545596801,-50182387208,

%T -314783998186522867201,-1259135992746091468808,

%U -198177931028585663493396958369763763148801,-792711724114342653973587833479055052595208

%N Optimal ascending continued fraction expansion of sqrt(44) - 6.

%C See A228929 for the definition of "optimal ascending continued fraction".

%C This is the first number whose expansion exhibits (in the first 20 terms) a different recurrence relation from that described in A228931.

%C Conjecture: The terms of the expansion of sqrt(x) are all negative starting from a(4) and satisfy these recurrence relations for n>=3: a(2n) = 4*a(2n-1) - 4 and a(2n+1) = -2*a(2n-1)^2 + 1.

%C Numbers (in the range 1..1000) that exhibit this recurrence starting from some n are 44, 125, 154, 160, 176, 207, 208, 280, 352, 384, 459, 468, 500, 608, 616, 640, 665, 686, 704, 768, 800, 832, 864, 874, 875, 924.

%H G. C. Greubel, <a href="/A228934/b228934.txt">Table of n, a(n) for n = 1..21</a>

%F a(2n) = 4*a(2n-1) - 4 and a(2n+1) = -2*a(2n-1)^2 + 1 for n >= 3.

%e sqrt(44) = 6 + 1/2*(1 + 1/4*(1 + 1/15*(1 - 1/99*(1 - 1/199*(1 - 1/800*(1 - 1/79201*(1 - 1/316808*(1 - 1/12545596801*(1 - ...))))))))).

%p ArticoExp := proc (n, q::posint)::list; local L, i, z; Digits := 50000; L := []; z := frac(evalf(n)); for i to q+1 do if z = 0 then break end if; L := [op(L), round(1/abs(z))*sign(z)]; z := abs(z)*round(1/abs(z))-1 end do; return L end proc

%p # List the first 20 terms of the expansion of sqrt(44)-6

%p ArticoExp(sqrt(44),20)

%t ArticoExp[x_, n_] := Round[1/#] & /@ NestList[Round[1/Abs[#]]*Abs[#] - 1 &, FractionalPart[x], n]; Block[{$MaxExtraPrecision = 50000}, ArticoExp[Sqrt[44] - 6, 20]] (* _G. C. Greubel_, Dec 26 2016 *)

%Y Cf. A228929, A228931, A228932.

%K sign,cofr

%O 1,1

%A _Giovanni Artico_, Sep 11 2013

%E Minor typos corrected by _Giovanni Artico_, Sep 24 2013