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A051009 Reduced denominators of Newton's iteration for sqrt(2). 10
1, 2, 12, 408, 470832, 627013566048, 1111984844349868137938112, 3497379255757941172020851852070562919437964212608 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For n>=4, A051009(n) = A098890(n-2) - A098890(n-3). - Kieren MacMillan, Dec 19 2007

(2^n)-th Pell numbers. - Sergio Falcon, Dec 04 2008

For n>1, egyptian fraction expansion of 2-sqrt(2), i.e., 2-sqrt(2) = 1/2 + 1/12 + 1/408 + 1/470832 + ... - Simon Plouffe, Feb 22 2011

LINKS

J. Conrad, Table of n, a(n) for n = 1..12

Eric Weisstein's World of Mathematics, Newton's Iteration

Eric Weisstein's World of Mathematics, Square Root.

Eric Weisstein's World of Mathematics, Pythagoras's Constant

FORMULA

a(n) = A000129(2^n).

a(n) = 2*a(n-1)*A001601(n-1). - Joe Keane (jgk(AT)jgk.org), May 31 2002

sqrt(2) = 1 + 1/2 - Sum_{n>=3} (1/a(n)). - Donald S. McDonald, Jan 21 2003

For n>1, a(n) = 2*a(n-1)*sqrt(2*a(n-1)^2+1). - Mario Catalani (mario.catalani(AT)unito.it), May 27 2003

For n>0: a(n) = Sum_{r=0..2^(n-1)-1} binomial(2^n, 2*r+1)*2^r. - Mario Catalani (mario.catalani(AT)unito.it), May 30 2003

a(n) = (1/(2*sqrt(2)))*((1 + sqrt(2))^(2^n) - (1 - sqrt(2))^(2^n)))), {n, 0, 7}]. - Artur Jasinski, Oct 10 2008

For n>0, a(n) = sqrt((A001601(n)^2-1)/2). - Jose Hortal, Apr 14 2012

a(1)=1, a(2)=2, a(n) = 2 * a(n-1) * cos(2^(n-3) * arccos(3)). - Daniel Suteu, Dec 01 2016

0 = a(n)^2*(2*a(n+1) + a(n+2)) - a(n+1)^3 if n>0. - Michael Somos, Dec 01 2016

a(n) = A001542(2^(n-2)). - A.H.M. Smeets, May 28 2017

EXAMPLE

G.f. = x + 2*x^2 + 12*x^3 + 408*x^4 + 470832*x^5 + ...

MATHEMATICA

Table[Simplify[Expand[(1/(2 Sqrt[2])) ((1 + Sqrt[2])^(2^n) - (1 - Sqrt[2])^(2^n))]], {n, 0, 7}] (* Artur Jasinski, Oct 10 2008 *)

Do[Print[Fibonacci[2^n, 2]], {n, 0, 10}] (* Sergio Falcon, Dec 04 2008 *)

CROSSREFS

Cf. A000129, A001601, A051009, A098890.

Sequence in context: A156509 A229919 A287679 * A324616 A060942 A072446

Adjacent sequences:  A051006 A051007 A051008 * A051010 A051011 A051012

KEYWORD

nonn,frac

AUTHOR

Eric W. Weisstein

STATUS

approved

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Last modified April 11 18:00 EDT 2021. Contains 342888 sequences. (Running on oeis4.)