A270546 - OEIS

%I #11 Feb 24 2018 14:00:44

%S 2,2,5,325,200533,65627675599,22975481891957121466348,

%T 1958997403653886589078102754522745217186637162,

%U 141280756113351994103874857935521871912536028357392961997286697261498102983722388787617517574

%N Denominators of r-Egyptian fraction expansion for sqrt(1/2), where r(k) = 1/(2k-1).

%C Suppose that r is a sequence of rational numbers r(k) <= 1 for k >= 1, and that x is an irrational number in (0,1).  Let f(0) = x, n(k) = floor(r(k)/f(k-1)), and f(k) = f(k-1) - r(k)/n(k).  Then x = r(1)/n(1) + r(2)/n(2) + r(3)/n(3) + ... , the r-Egyptian fraction for x.

%C See A269993 for a guide to related sequences.

%H Clark Kimberling, <a href="/A270546/b270546.txt">Table of n, a(n) for n = 1..12</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/EgyptianFraction.html">Egyptian Fraction</a>

%H <a href="/index/Ed#Egypt">Index entries for sequences related to Egyptian fractions</a>

%e sqrt(1/2) = 1/(1*2) + 1/(3*2) + 1/(5*5) + 1/(7*325) + ...

%t r[k_] := 1/(2k-1); f[x_, 0] = x; z = 10;

%t n[x_, k_] := n[x, k] = Ceiling[r[k]/f[x, k - 1]]

%t f[x_, k_] := f[x, k] = f[x, k - 1] - r[k]/n[x, k]

%t x = Sqrt(1/2); Table[n[x, k], {k, 1, z}]

%o (PARI) r(k) = 1/(2*k-1);

%o f(k,x) = if (k==0, x, f(k-1, x) - r(k)/a(k, x););

%o a(k, x=sqrt(1/2)) = ceil(r(k)/f(k-1, x)); \\ _Michel Marcus_, Apr 03 2016

%Y Cf. A269993, A005408, A010503.

%K nonn,frac,easy

%O 1,1

%A _Clark Kimberling_, Apr 02 2016