# Nested radical

In algebra, a **nested radical** is a radical expression that contains another radical expression. Examples include:

which arises in discussing the regular pentagon;

or more complicated ones such as:

## Denesting nested radicals

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 process is generally considered a difficult problem, although a special class of nested radical can be denested by assuming it denests into a sum of two surds:

Squaring both sides of this equation yields:

This can be solved by using the quadratic formula and setting rational and irrational parts on both sides of the equation equal to each other. The solutions for *e* and *d* can be obtained by first equating the rational parts:

which gives

For the irrational parts note that

and squaring both sides yields

By plugging in *a* − *e* for *d* one obtains

Rearranging terms will give a quadratic equation which can be solved for *e*:

The solution *d* is the algebraic conjugate of *e*. If

then

However, this approach works for nested radicals of the form if and only if is an integer, in which case the nested radical can be denested into a sum of surds.

In some cases, higher-power radicals may be needed to denest the nested radical.

### Some identities of Ramanujan

Srinivasa Ramanujan demonstrated a number of curious identities involving denesting of radicals. Among them are the following:^{[1]}

Other odd-looking radicals inspired by Ramanujan:

## Landau's algorithm

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In 1989 Susan Landau introduced the first algorithm for deciding which nested radicals can be denested.^{[3]} Earlier algorithms worked in some cases but not others.

## Infinitely nested radicals

### Square roots

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 real root of the equation *x*^{2} − *x* − *n* = 0. For *n* = 1, this root is the golden ratio φ, approximately equal to 1.618. The same procedure also works to get that

and is the real root of the equation *x*^{2} + *x* − *n* = 0. For *n* = 1, this root is the reciprocal of the golden ratio Φ, which is equal to φ − 1. This method will give a rational *x* value for all values of *n* such that

Ramanujan posed this 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:

Ramanujan stated this radical in his lost notebook

(The repeating pattern of the signs is

### Cube roots

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 real root of the equation *x*^{3} − *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 *x*^{3} + *x* − *n* = 0 for all *n* and *x* where *n* > 0 and |*x*| ≥ 1.

## See also

## References

- ↑ "A note on 'Zippel Denesting'", Susan Landau, http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.35.5512&rep=rep1&type=pdf
- ↑ "RADICALS AND UNITS IN RAMANUJAN’S WORK", Susan Landau, http://www.math.uiuc.edu/~berndt/articles/radicals.ps
- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}

### Further reading

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