Nested radical

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In algebra, a nested radical is a radical expression (one containing a square root sign, cube root sign, etc.) that contains (nests) another radical expression. Examples include

which arises in discussing the regular pentagon, and more complicated ones such as

Denesting[edit]

Some nested radicals can be rewritten in a form that is not nested. For example,

Rewriting a nested radical in this way is called denesting. This is not always possible, and, even when possible, it is often difficult.

Two nested square roots[edit]

In the case of two nested square roots, the following theorem completely solves the problem of denesting.[1]

If a, b, and c are rational numbers and c is not a square, there are two rational numbers x and y such that

if and only if is the square of a rational number d. If the nested radical is real, x and y are the two numbers In particular, if a, b, and c are integers, then 2x and 2y are integers.

This result includes denestings of the form

as z may always be written and at least one of the terms must be positive (because the left-hand side of the equation is positive).

A more general denesting formula could have the form

However, Galois theory implies that either the left-and side belongs to or it must be obtained by changing the sign of either or both. In the first case, this means that one can take x = c and In the second case, and another coefficient must be zero. If , one may rename xy as x for getting . Proceeding similarly if it results that one can suppose This shows that the apparently more general denesting can always be reduced to the above one.

Proof: By squaring, the equation

is equivalent with

and, in the case of a minus in the right-hand side,

|x||y|,

(square roots are nonnegative by definition of the notation). As the inequality may always be satisfied by possibly exchanging x and y, solving the first equation in x and y is equivalent with solving

This equality implies that belongs to the quadratic field In this field every element may be uniquely written with and being rational numbers. This implies that is not rational (otherwise, the left-hand side of the equation would be irrational and the right-hand side would be rational). As x and y must be rational, the square of must be rational. This implies that in the expression of as Thus

for some rational number The uniqueness of the decomposition over 1 and implies thus that the equation that is considered is equivalent with

It follows by Vieta's formulas that x and y must be the roots of the quadratic equation

that is

Thus x and y are rational if and only if is a rational number.

For explicitly choosing the various signs, one must consider only positive real square roots, and thus assuming c > 0. The equation shows that |a| > |b|c. Thus, if the nested radical is real, and denesting is possible, then a > 0. Then, the two possible cases for the sign of b may be considered simultaneously, by assuming b > 0 and writing the solution

Some identities of Ramanujan[edit]

Srinivasa Ramanujan demonstrated a number of curious identities involving nested radicals. Among them are the following:[2]

[3]

Other odd-looking radicals inspired by Ramanujan include:

Landau's algorithm[edit]

In 1989 Susan Landau introduced the first algorithm for deciding which nested radicals can be denested.[4] Earlier algorithms worked in some cases but not others.

In trigonometry[edit]

In trigonometry, the sines and cosines of many angles can be expressed in terms of nested radicals. For example,

and

The last equality results directly from the results of § Two square roots.

In the solution of the cubic equation[edit]

Nested radicals appear in the algebraic solution of the cubic equation. Any cubic equation can be written in simplified form without a quadratic term, as

whose general solution for one of the roots is

In the case in which the cubic has only one real root, the real root is given by this expression with the radicands of the cube roots being real and with the cube roots being the real cube roots. In the case of three real roots, the square root expression is an imaginary number; here any real root is expressed by defining the first cube root to be any specific complex cube root of the complex radicand, and by defining the second cube root to be the complex conjugate of the first one. The nested radicals in this solution cannot in general be simplified unless the cubic equation has at least one rational solution. Indeed, if the cubic has three irrational but real solutions, we have the casus irreducibilis, in which all three real solutions are written in terms of cube roots of complex numbers. On the other hand, consider the equation

which has the rational solutions 1, 2, and −3. The general solution formula given above gives the solutions

For any given choice of cube root and its conjugate, this contains nested radicals involving complex numbers, yet it is reducible (even though not obviously so) to one of the solutions 1, 2, or –3.

Infinitely nested radicals[edit]

Square roots[edit]

Under certain conditions infinitely nested square roots such as

represent rational numbers. This rational number can be found by realizing that x also appears under the radical sign, which gives the equation

If we solve this equation, we find that x = 2 (the second solution x = −1 doesn't apply, under the convention that the positive square root is meant). This approach can also be used to show that generally, if n > 0, then

and is the positive root of the equation x2 − x − n = 0. For n = 1, this root is the golden ratio φ, approximately equal to 1.618. The same procedure also works to obtain, if n > 1,

which is the positive root of the equation x2 + x − n = 0.

Ramanujan's infinite radicals[edit]

Ramanujan posed the following problem to the Journal of Indian Mathematical Society:

This can be solved by noting a more general formulation:

Setting this to F(x) and squaring both sides gives us

which can be simplified to

It can then be shown that

So, setting a = 0, n = 1, and x = 2, we have

Ramanujan stated the following infinite radical denesting in his lost notebook:

The repeating pattern of the signs is

Viète's expression for π[edit]

Viète's formula for π, the ratio of a circle's circumference to its diameter, is

Cube roots[edit]

In certain cases, infinitely nested cube roots such as

can represent rational numbers as well. Again, by realizing that the whole expression appears inside itself, we are left with the equation

If we solve this equation, we find that x = 2. More generally, we find that

is the positive real root of the equation x3 − x − n = 0 for all n > 0. For n = 1, this root is the plastic number ρ, approximately equal to 1.3247.

The same procedure also works to get

as the real root of the equation x3 + x − n = 0 for all n > 1.

See also[edit]

References[edit]

  1. ^ Euler, Leonhard (2012). Elements of algebra. Springer Science & Business Media. Chapter VIII.
  2. ^ Landau, Susan (1993). "A note on 'Zippel Denesting'". CiteSeerX 10.1.1.35.5512. Cite journal requires |journal= (help)
  3. ^ Berndt, Bruce; Chan, Heng; Zhang, Liang-Cheng (1998). "Radicals and units in Ramanujan's work" (PDF). Acta Arithmetica. 87 (2): 145–158. doi:10.4064/aa-87-2-145-158.
  4. ^ Landau, Susan (1992). Simplification of Nested Radicals. Journal of Computation. 21. SIAM. pp. 85–110. CiteSeerX 10.1.1.34.2003. doi:10.1109/SFCS.1989.63496. ISBN 978-0-8186-1982-3.

Further reading[edit]