# Palindromic polynomial

A polynomial is palindromic, if the sequence of its coefficients are a palindrome.

Let

$P(x) = \sum_{i=0}^n a_ix^i$

be a polynomial of degree n, then P is palindromic if ai = ani for i = 0, 1, ... n.

Similarly, P is called antipalindromic if ai = −ani for i = 0, 1, ... n. It follows from the definition that if P is of even degree (so has odd number of terms in the polynomial), then it can only be antipalindromic when the 'middle' term is 0, i.e. ai = −an, where n = 2i.

## Examples

Some examples of palindromic polynomials are:

$(x+1)^2 = x^2 + 2x + 1$
$(x+1)^3 = x^3 + 3x^2 + 3x + 1.$

These are examples of the expansion of (x + 1)n, which is palindromic for all n, this can be seen from the binomial expansion.

Another example of a palindromic polynomial [which isn't of the form (x + 1)n] is:

$x^2 + 3x + 1$

An example of an antipalindromic polynomial is:

$x^2 - 1$

Note the zero coefficient for the term in x.

## Properties

1. If a is a root of a polynomial that is either palindromic or antipalindromic, then 1/a is also a root and has the same multiplicity.[1]
2. The converse is true: If a polynomial is such that if a is a root then 1/a is also a root of the same multiplicity, then the polynomial is either palindromic or antipalindromic.
3. The product of two palindromic or antipalindromic polynomials is palindromic.
4. The product of a palindromic polynomial and an antipalindromic polynomial is antipalindromic.
5. A palindromic polynomial of odd degree is a multiple of x+1 (it has -1 as a root) and its quotient by x+1 is also palindromic.
6. An antipalindromic polynomial is a multiple of x-1 (it has 1 as a root) and its quotient by x-1 is palindromic.
7. An antipalindromic polynomial of even degree is a multiple of x2-1 (it has -1 and 1 as a roots) and its quotient by x2-1 is palindromic.
8. If p(x) is a palindromic polynomial of even degree 2d, then there is a polynomial q of degree d such that xdq(x+1/x) = p(x).

It results from these properties that the study of the roots of a polynomial of degree d that is either palindromic or antipalindromic may be reduced to the study of the roots of a polynomial of degree at most d/2.

## Factorization

Factorization techniques (and the search for roots) follow on directly from the properties listed above.

For example, Property 5 yields an immediate factor x+1 for palindromic polynomials of odd degree.

As another example, Property 8 leads to the technique of dividing by xd and replacing x + 1/x by X.

As an example of the latter technique suppose

$x^4 + x^2 + 1 = 0$

Letting X = x + 1/x, dividing by x2 and deriving

$X^2 = x^2 + 2 + 1/x^2$

we have the much simpler

$X^2 - 1 = 0$

which factorizes as

$(X - 1)(X + 1) = 0$

so either X = 1 or X = - 1

The X = - 1 case yields

$x + 1/x = - 1$

or

$x^2 + x + 1 = 0$

which has no real roots.

The X = 1 case yields

$x + 1/x = 1$

or

$x^2 - x + 1 = 0$

which also has no real roots.

## Converting other polynomials to palindromic form

Some polynomials can be converted to palindromic form by, for example, suitable substutions. For example consider

$4x^2 + 4x + 1$.

Writing y = 2x this becomes

$y^2 + 2y + 1$

or

$(y + 1)^2$

with the resultant factorization

$(2x + 1)^2$

Similar techniques might yield a polynomial in antipalindromic form.