%I #40 Dec 27 2016 02:35:27

%S 2,9,30,60,122,-878,11429,35241,-177141,709582,-3123032,-1157723745,

%T 3237738813,-16178936725,33395053634,-71863018424,-153349368674,

%U -386763022623,-8021033029400,16314606875900,52522689388692

%N Optimal ascending continued fraction expansion of sqrt(43) - 6.

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

%C In A228931 it is shown that many numbers of the type sqrt(x) seem to present in their expansion a recurrence relation a(n) = a(n-1)^2 - 2 between the terms, starting from some point onward; 43 is the first natural number whose terms don't respect this relation.

%C The numbers in range 1 .. 200 that exhibit this behavior are 43, 44, 46, 53, 58, 61, 67, 73, 76, 85, 86, 89, 91, 94, 97, 103, 106, 108, 109, 113, 115, 116, 118, 125, 127, 129, 131, 134, 137, 139, 149, 151, 153, 154, 157, 159, 160, 161, 163, 166, 172, 173, 176, 177, 179, 181, 184, 186, 190, 191, 193, 199.

%C Nevertheless, the expansions of 3*sqrt(43), 9*sqrt(43), and sqrt(43)/5 satisfy the recurrence relation.

%D See A228931.

%H G. C. Greubel, <a href="/A228932/b228932.txt">Table of n, a(n) for n = 1..500</a>

%e sqrt(43) = 6 + 1/2*(1 + 1/9*(1 + 1/30*(1 + 1/60*(1 + 1/122*(1 - 1/878*(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 8 terms of the expansion of sqrt(43)-6

%p ArticoExp(sqrt(43),20)

%t ArticoExp[x_, n_] := Round[1/#] & /@ NestList[Round[1/Abs[#]]*Abs[#] - 1 &, FractionalPart[x], n]; Block[{$MaxExtraPrecision = 50000},

%t ArticoExp[Sqrt[43] - 6, 20]] (* _G. C. Greubel_, Dec 26 2016 *)

%Y Cf. A010134, A010497, A228929, A228931.

%K sign,cofr

%O 1,1

%A _Giovanni Artico_, Sep 10 2013