Reciprocal polynomial

with coefficients from an arbitrary field, its reciprocal polynomial or reflected polynomial,[1][2] denoted by $p *$ or $p R$ ,[2][1] is the polynomial[3]

p^{*}(x)=a_{n}+a_{n-1}x+\cdots +a_{0}x^{n}=x^{n}p(x^{-1}).

In algebra, given a polynomial

p(x)=a_{0}+a_{1}x+a_{2}x^{2}+\cdots +a_{n}x^{n},

That is, the coefficients of $p *$ are the coefficients of $p$ in reverse order. They arise naturally in linear algebra as the characteristic polynomial of the inverse of a matrix.

In the special case where the field is the complex numbers, when

p(z)=a_{0}+a_{1}z+a_{2}z^{2}+\cdots +a_{n}z^{n},

the conjugate reciprocal polynomial, denoted $p †$ , is defined by,

p^{\dagger }(z)={\overline {a_{n}}}+{\overline {a_{n-1}}}z+\cdots +{\overline {a_{0}}}z^{n}=z^{n}{\overline {p({\bar {z}}^{-1})}},

where ${\overline {a_{i}}}$ denotes the complex conjugate of $a_{i}$ , and is also called the reciprocal polynomial when no confusion can arise.

A polynomial $p$ is called self-reciprocal or palindromic if $p (x) = p * (x)$ . The coefficients of a self-reciprocal polynomial satisfy $a i = a n - i$ for all $i$ . In the conjugate reciprocal case, the coefficients must be real to satisfy the condition.

Properties

Reciprocal polynomials have several connections with their original polynomials, including:

$deg p = deg p *$
$p (x) = x n p * (x -1)$ [2]
$α$ is a root of a polynomial $p$ if and only if $α -1$ is a root of $p *$ .[4]
If $p (x) \neq x$ then $p$ is irreducible if and only if $p *$ is irreducible.[5]
$p$ is primitive if and only if $p *$ is primitive.[4]

Other properties of reciprocal polynomials may be obtained, for instance:

If a polynomial is self-reciprocal and irreducible then it must have even degree.[5]

Palindromic and antipalindromic polynomials

A self-reciprocal polynomial is also called palindromic because its coefficients, when the polynomial is written in the order of ascending or descending powers, form a palindrome. That is, if

P(x)=\sum _{i=0}^{n}a_{i}x^{i}

is a polynomial of degree $n$ , then $P$ is palindromic if $a i = a n - i$ for $i = 0, 1, ..., n$ . Some authors use the terms palindromic and reciprocal interchangeably.

Similarly, a polynomial $P$ of degree $n$ is called antipalindromic if $a i = - a n - i$ for $i = 0, 1, ..., n$ . That is, a polynomial $P$ is antipalindromic if $P (x) = - P * (x)$ .

Examples

From the properties of the binomial coefficients, it follows that the polynomials $P (x) = (x + 1) n$ are palindromic for all positive integers $n$ , while the polynomials $Q (x) = (x - 1) n$ are palindromic when $n$ is even and antipalindromic when $n$ is odd.

Other examples of palindromic polynomials include cyclotomic polynomials and Eulerian polynomials.

Properties

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.[6]
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.
For any polynomial $q$ , the polynomial $q + q *$ is palindromic and the polynomial $q - q *$ is antipalindromic.
It follows that any polynomial $q$ can be written as the sum of a palindromic and an antipalindromic polynomial, since $q = (q + q *)/2 + (q - q *)/2$ .[7]
The product of two palindromic or antipalindromic polynomials is palindromic.
The product of a palindromic polynomial and an antipalindromic polynomial is antipalindromic.
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.
An antipalindromic polynomial is a multiple of $x - 1$ (it has 1 as a root) and its quotient by $x - 1$ is palindromic.
An antipalindromic polynomial of even degree is a multiple of $x 2 - 1$ (it has −1 and 1 as a roots) and its quotient by $x 2 - 1$ is palindromic.
If $p (x)$ is a palindromic polynomial of even degree 2 $d$ , then there is a polynomial $q$ of degree $d$ such that $p (x) = x d q (x + 1 / x)$ (Durand 1961).
If $p (x)$ is a monic antipalindromic polynomial of even degree 2 $d$ over a field $k$ with odd characteristic, then it can be written uniquely as $p (x) = x d (Q (x) - Q (1 / x))$ , where $Q$ is a monic polynomial of degree $d$ with no constant term.[8]
If an antipalindromic polynomial $P$ has even degree $2 n$ , then its "middle" coefficient (of power $n$ ) is 0 since $a n = - a 2 n - n$ .

Real coefficients

A polynomial with real coefficients all of whose complex roots lie on the unit circle in the complex plane (that is, all the roots have modulus 1) is either palindromic or antipalindromic.[9]

Conjugate reciprocal polynomials

A polynomial is conjugate reciprocal if $p(x)\equiv p^{\dagger }(x)$ and self-inversive if $p(x)=\omega p^{\dagger }(x)$ for a scale factor $ω$ on the unit circle.[10]

If $p (z)$ is the minimal polynomial of $z 0$ with $| z 0 | = 1, z 0 \neq 1$ , and $p (z)$ has real coefficients, then $p (z)$ is self-reciprocal. This follows because

z_{0}^{n}{\overline {p(1/{\bar {z_{0}}})}}=z_{0}^{n}{\overline {p(z_{0})}}=z_{0}^{n}{\bar {0}}=0.

So $z 0$ is a root of the polynomial $z^{n}{\overline {p({\bar {z}}^{-1})}}$ which has degree $n$ . But, the minimal polynomial is unique, hence

cp(z)=z^{n}{\overline {p({\bar {z}}^{-1})}}

for some constant $c$ , i.e. $ca_{i}={\overline {a_{n-i}}}=a_{n-i}$ . Sum from $i = 0$ to $n$ and note that 1 is not a root of $p$ . We conclude that $c = 1$ .

A consequence is that the cyclotomic polynomials $Φ n$ are self-reciprocal for $n > 1$ . This is used in the special number field sieve to allow numbers of the form $x 11 \pm 1, x 13 \pm 1, x 15 \pm 1$ and $x 21 \pm 1$ to be factored taking advantage of the algebraic factors by using polynomials of degree 5, 6, 4 and 6 respectively – note that $φ$ (Euler's totient function) of the exponents are 10, 12, 8 and 12.

Application in coding theory

The reciprocal polynomial finds a use in the theory of cyclic error correcting codes. Suppose $x n - 1$ can be factored into the product of two polynomials, say $x n - 1 = g (x) p (x)$ . When $g (x)$ generates a cyclic code $C$ , then the reciprocal polynomial $p *$ generates $C ⊥$ , the orthogonal complement of $C$ .[11] Also, $C$ is self-orthogonal (that is, $C \subseteq C ⊥)$ , if and only if $p *$ divides $g (x)$ .[12]

Notes

- Graham, Ronald; Knuth, Donald E.; Patashnik, Oren (1994). Concrete mathematics : a foundation for computer science (Second ed.). Reading, Mass: Addison-Wesley. p. 340. ISBN 978-0201558029.
Aigner, Martin (2007). A course in enumeration. Berlin New York: Springer. p. 94. ISBN 978-3540390329.
Roman 1995, pg.37
Pless 1990, pg. 57
Roman 1995, pg. 37
Pless 1990, pg. 57 for the palindromic case only
Stein, Jonathan Y. (2000), Digital Signal Processing: A Computer Science Perspective, Wiley Interscience, p. 384, ISBN 9780471295464
Katz, Nicholas M. (2012), Convolution and Equidistribution : Sato-Tate Theorems for Finite Field Mellin Transformations, Princeton University Press, p. 146, ISBN 9780691153315
Markovsky, Ivan; Rao, Shodhan (2008), "Palindromic polynomials, time-reversible systems and conserved quantities" (PDF), Control and Automation: 125–130, doi:10.1109/MED.2008.4602018, ISBN 978-1-4244-2504-4
Sinclair, Christopher D.; Vaaler, Jeffrey D. (2008). "Self-inversive polynomials with all zeros on the unit circle". In McKee, James; Smyth, C. J. (eds.). Number theory and polynomials. Proceedings of the workshop, Bristol, UK, April 3–7, 2006. London Mathematical Society Lecture Note Series. 352. Cambridge: Cambridge University Press. pp. 312–321. ISBN 978-0-521-71467-9. Zbl 1334.11017.
Pless 1990, pg. 75, Theorem 48
Pless 1990, pg. 77, Theorem 51

References

Pless, Vera (1990), Introduction to the Theory of Error Correcting Codes (2nd ed.), New York: Wiley-Interscience, ISBN 0-471-61884-5
Roman, Steven (1995), Field Theory, New York: Springer-Verlag, ISBN 0-387-94408-7
Émile Durand (1961) Solutions numériques des équations algrébriques I, Masson et Cie: XV - polynômes dont les coefficients sont symétriques ou antisymétriques, p. 140-141.

External links

"The Fundamental Theorem for Palindromic Polynomials". MathPages.com.
Reciprocal Polynomial (on MathWorld)

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[concrete-1] Graham, Ronald; Knuth, Donald E.; Patashnik, Oren (1994). Concrete mathematics : a foundation for computer science (Second ed.). Reading, Mass: Addison-Wesley. p. 340. ISBN 978-0201558029.

[2] Graham, Ronald; Knuth, Donald E.; Patashnik, Oren (1994). Concrete mathematics : a foundation for computer science (Second ed.). Reading, Mass: Addison-Wesley. p. 340. ISBN 978-0201558029.

[Aigner-2] Aigner, Martin (2007). A course in enumeration. Berlin New York: Springer. p. 94. ISBN 978-3540390329.

[3] Roman 1995, pg.37

[Pless_1990_loc=pg._57-4] Pless 1990, pg. 57

[Roman_1995_loc=_pg._37-5] Roman 1995, pg. 37

[6] Pless 1990, pg. 57 for the palindromic case only

[7] Stein, Jonathan Y. (2000), Digital Signal Processing: A Computer Science Perspective, Wiley Interscience, p. 384, ISBN 9780471295464

[8] Katz, Nicholas M. (2012), Convolution and Equidistribution : Sato-Tate Theorems for Finite Field Mellin Transformations, Princeton University Press, p. 146, ISBN 9780691153315

[9] Markovsky, Ivan; Rao, Shodhan (2008), "Palindromic polynomials, time-reversible systems and conserved quantities" (PDF), Control and Automation: 125–130, doi:10.1109/MED.2008.4602018, ISBN 978-1-4244-2504-4

[SV08-10] Sinclair, Christopher D.; Vaaler, Jeffrey D. (2008). "Self-inversive polynomials with all zeros on the unit circle". In McKee, James; Smyth, C. J. (eds.). Number theory and polynomials. Proceedings of the workshop, Bristol, UK, April 3–7, 2006. London Mathematical Society Lecture Note Series. 352. Cambridge: Cambridge University Press. pp. 312–321. ISBN 978-0-521-71467-9. Zbl 1334.11017.

[11] Pless 1990, pg. 75, Theorem 48

[12] Pless 1990, pg. 77, Theorem 51

Reciprocal polynomial

Properties

Palindromic and antipalindromic polynomials

Examples

Properties

Real coefficients

Conjugate reciprocal polynomials

Application in coding theory

See also

Notes

References

External links