TOPICS
Search

Nested Radical

DOWNLOAD Mathematica NotebookDownload Wolfram Notebook

Expressions of the form

lim_(k->infty)x_0+sqrt(x_1+sqrt(x_2+sqrt(...+x_k)))
(1)

are called nested radicals. Herschfeld (1935) proved that a nested radical of real nonnegative terms converges iff (x_n)^(2^(-n)) is bounded. He also extended this result to arbitrary powers (which include continued square roots and continued fractions as well), a result is known as Herschfeld's convergence theorem.

Nested radicals appear in the computation of pi,

2/pi=sqrt(1/2)sqrt(1/2+1/2sqrt(1/2))sqrt(1/2+1/2sqrt(1/2+1/2sqrt(1/2)))...
(2)

(Vieta 1593; Wells 1986, p. 50; Beckmann 1989, p. 95), in trigonometrical values of cosine and sine for arguments of the form pi/2^n, e.g.,

sin(pi/8)=1/2sqrt(2-sqrt(2))
(3)
cos(pi/8)=1/2sqrt(2+sqrt(2))
(4)
sin(pi/(16))=1/2sqrt(2-sqrt(2+sqrt(2)))
(5)
cos(pi/(16))=1/2sqrt(2+sqrt(2+sqrt(2))).
(6)

Nest radicals also appear in the computation of the golden ratio

phi=sqrt(1+sqrt(1+sqrt(1+sqrt(1+...))))
(7)

and plastic constant

P=RadicalBox[{1, +, RadicalBox[{1, +, RadicalBox[{1, +, ...}, 3]}, 3]}, 3].
(8)

Both of these are special cases of

x=RadicalBox[{a, +, RadicalBox[{a, +, ...}, n]}, n],
(9)

which can be exponentiated to give

x^n=a+RadicalBox[{a, +, RadicalBox[{a, +, ...}, n]}, n],
(10)

so solutions are

x^n=a+x.
(11)

In particular, for n=2, this gives

x=1/2(1+sqrt(4a+1)).
(12)

The silver constant is related to the nested radical expression

RadicalBox[{7, +, 7, RadicalBox[{7, +, ...}, 3]}, 3].
(13)

There are a number of general formula for nested radicals (Wong and McGuffin). For example,

x=RadicalBox[{{(, {1, -, q}, )}, {x, ^, n}, +, q, {x, ^, {(, {n, -, 1}, )}}, RadicalBox[{{(, {1, -, q}, )}, {x, ^, n}, +, q, {x, ^, {(, {n, -, 1}, )}}, RadicalBox[..., n]}, n]}, n]
(14)

which gives as special cases

(b+sqrt(b^2+4a))/2=sqrt(a+bsqrt(a+bsqrt(a+bsqrt(...))))
(15)

(n=2, q=1-a/x^2, x=b/q),

x=RadicalBox[{{x, ^, {(, {n, -, 1}, )}}, RadicalBox[{{x, ^, {(, {n, -, 1}, )}}, RadicalBox[{{x, ^, {(, {n, -, 1}, )}}, RadicalBox[..., n]}, n]}, n]}, n]
(16)

(q=1), and

x=sqrt(xsqrt(xsqrt(xsqrt(xsqrt(...)))))
(17)

(q=1,n=2). Equation (14) also gives rise to

q^((n^k-1)/(n-1))x^(n^j)=RadicalBox[{{q, ^, {(, {{(, {{n, ^, {(, {k, +, 1}, )}}, -, n}, )}, /, {(, {n, -, 1}, )}}, )}}, {(, {1, -, q}, )}, {x, ^, {(, {n, ^, {(, {j, +, 1}, )}}, )}}, +, ...}, n] ...+RadicalBox[{{q, ^, {(, {{(, {{n, ^, {(, {k, +, 2}, )}}, -, n}, )}, /, {(, {n, -, 1}, )}}, )}}, {(, {1, -, q}, )}, {x, ^, {(, {n, ^, {(, {j, +, 2}, )}}, )}}, +, RadicalBox[..., n]}, n]^_,
(18)

which gives the special case for q=1/2, n=2, x=1, and k=-1,

sqrt(2)=sqrt(2/(2^(2^0))+sqrt(2/(2^(2^1))+sqrt(2/(2^(2^2))+sqrt(2/(2^(2^3))+sqrt(2/(2^(2^4))+...))))).
(19)

Equation (◇) can be generalized to

x^(1/(n-1))=RadicalBox[{x, RadicalBox[{x, RadicalBox[{x, ...}, n]}, n]}, n]
(20)

for integers n>=2, which follows from

1+1/n+1/(n^2)+...=1/(1-1/n)
(21)
=n/(n-1)
(22)
=1+1/(n-1)
(23)
1/n+1/(n^2)+1/(n^3)+...=1/(n-1)
(24)
1/n(1+1/n(1+1/n(1+...)))=1/(n-1).
(25)

In particular, taking n=3 gives

sqrt(x)=RadicalBox[{x, RadicalBox[{x, RadicalBox[{x, ...}, 3]}, 3]}, 3].
(26)

(J. R. Fielding, pers. comm., Oct. 8, 2002).

Ramanujan discovered

x+n+a=sqrt(ax+(n+a)^2+xsqrt(a(x+n)+(n+a)^2+...)) ...+(x+n)sqrt(a(x+2n)+(n+a)^2+(x+2n)sqrt(...))^_^_,
(27)

which gives the special cases

x+1=sqrt(1+xsqrt(1+(x+1)sqrt(1+(x+2)sqrt(1+...))))
(28)

for a=0, n=1 (Ramanujan 1911; Ramanujan 2000, p. 323; Pickover 2002, p. 310), and

3=sqrt(1+2sqrt(1+3sqrt(1+4sqrt(1+5sqrt(...)))))
(29)

for a=0, n=1, and x=2. The justification of this process in general (and in the particular example of lnsigma, where sigma is Somos's quadratic recurrence constant) is given by Vijayaraghavan (in Ramanujan 2000, p. 348).

An amusing nested radical follows rewriting the series for e

e=1+1/(1!)+1/(2!)+1/(3!)+...
(30)

as

e=1+1+1/2(1+1/3(1+1/4(1+1/5(1+...)))),
(31)

so

x^(e-2)=sqrt(xRadicalBox[{x, RadicalBox[{x, RadicalBox[{x, ...}, 5]}, 4]}, 3])
(32)

(J. R. Fielding, pers. comm., May 15, 2002).

See also

Bolyai Expansion, Continued Fraction, Golden Ratio, Herschfeld's Convergence Theorem, Nested Radical Constant, Paris Constant, Pi Formulas, Power Tower, Ramanujan Log-Trigonometric Integrals, Silver Constant, Somos's Quadratic Recurrence Constant, Square Root

Explore with Wolfram|Alpha

References

Beckmann, P. A History of Pi, 3rd ed. New York: Dorset Press, 1989.Berndt, B. C. Ramanujan's Notebooks, Part IV. New York: Springer-Verlag, pp. 14-20, 1994.Borwein, J. M. and de Barra, G. "Nested Radicals." Amer. Math. Monthly 98, 735-739, 1991.Herschfeld, A. "On Infinite Radicals." Amer. Math. Monthly 42, 419-429, 1935.Jeffrey, D. J. and Rich, A. D. In Computer Algebra Systems (Ed. M. J. Wester). Chichester, England: Wiley, 1999.Landau, S. "A Note on 'Zippel Denesting.' " J. Symb. Comput. 13, 31-45, 1992a.Landau, S. "Simplification of Nested Radicals." SIAM J. Comput. 21, 85-110, 1992b.Landau, S. "How to Tangle with a Nested Radical." Math. Intell. 16, 49-55, 1994.Landau, S. "sqrt(2)+sqrt(3): Four Different Views." Math. Intell. 20, 55-60, 1998.Pólya, G. and Szegö, G. Problems and Theorems in Analysis, Vol. 1. New York: Springer-Verlag, 1997.Ramanujan, S. Question No. 298. J. Indian Math. Soc. 1911.Ramanujan, S. Collected Papers of Srinivasa Ramanujan (Ed. G. H. Hardy, P. V. S. Aiyar, and B. M. Wilson). Providence, RI: Amer. Math. Soc., p. 327, 2000.Sizer, W. S. "Continued Roots." Math. Mag. 59, 23-27, 1986.Vieta, F. Uriorum de rebus mathematicis responsorum. Liber VII. 1593. Reprinted in New York: Georg Olms, pp. 398-400 and 436-446, 1970.Wells, D. The Penguin Dictionary of Curious and Interesting Numbers. Middlesex, England: Penguin Books, 1986.Wong, B. and McGuffin, M. "The Museum of Infinite Nested Radicals." http://www.dgp.toronto.edu/~mjmcguff/math/nestedRadicals.html.

Referenced on Wolfram|Alpha

Nested Radical

Cite this as:

Weisstein, Eric W. "Nested Radical." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/NestedRadical.html

Subject classifications