# RootApproximant

converts the number x to one of the "simplest" algebraic numbers that approximates it well.

RootApproximant[x,n]

finds an algebraic number of degree at most n that approximates x.

# Details and Options • For degrees above 2, RootApproximant generates Root objects.
• effectively tests the total number of bits in the description of x by successively higher-degree algebraic numbers, and returns the first case for which the number of bits is small.
• Results from RootApproximant may not be unique.
• MinimalPolynomial yields the minimal polynomial for the result of RootApproximant.
• The option Method->{"DegreeCost"->p} specifies an additional cost p to be used for each successively higher power in determining the "simplest" approximation.

# Examples

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## Basic Examples(2)

Find a quadratic approximation to :

Find algebraic approximants of any order:

## Scope(4)

There is no simple quadratic approximation to N :

At degree 4, there is a simple answer, which turns out to be equal to :

Machine precision is not enough to recover the Root object equal to :

30 digits of precision suffice to recover the exact value here:

When given exact input, RootApproximant uses machine-precision approximations:

RootApproximant works with complex numbers:

## Options(1)

### Method(1)

Assigning additional cost to higher powers can be used to lower the degree of result:

## Applications(2)

Find successive algebraic approximations to :

Use FindRoot to find the real root near 2.5:

Use RootApproximant to find an algebraic number close to the root:

Check whether the result is a root of the original expression:

## Properties & Relations(3)

RootApproximant gives a Root object close to a given algebraic number:

The Root object found may not be exactly equal to the input:

Use RootReduce to find exact Root object representations of algebraic numbers:

Specifying a linear polynomial effectively finds a rational approximation to x:

Rationalize also gives a rational approximation, but it need not be the same:

Interestingly, the approximations can be found among continued fraction convergents: Use LatticeReduce to recognize linear combinations of more general functions:

The final relationship :

## Possible Issues(1)

Recognizing an algebraic number may require using higher precision:

The result is not equal to a:

Providing the correct degree improves the chances of recognizing an algebraic number:

A penalty may be used to lower the degree; here it does not help in recognizing the number:

Using a higher-precision approximation allows the algebraic number to be recognized: