Generalized permutation matrix: Difference between revisions

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{{Short description|Matrix with one nonzero entry in each row and column}}

{{unreferenced|date=August 2009}}

In [[mathematics]], a '''generalized permutation matrix''' (or '''monomial matrix''') is a [[matrix (mathematics)|matrix]] with the same nonzero pattern as a [[permutation matrix]], i.e. there is exactly one nonzero entry in each row and each column. Unlike a permutation matrix, where the nonzero entry must be 1, in a generalized permutation matrix the nonzero entry can be any nonzero value. An example of a generalized permutation matrix is

:<math>\begin{bmatrix}

0 & 0 & 3 & 0\\

0 & -27 & 0 & 0\\

1 & 0 & 0 & 0\\

0 & 0 & 0 & 1\sqrt2\end{bmatrix}.</math>

== Structure ==

An [[invertible matrix]] ''A'' is a generalized permutation matrix [[if and only if]] it can be written as a product of an [[invertible matrix|invertible]] [[diagonal matrix]] ''D'' and an (implicitly [[invertible matrix|invertible]]) [[permutation matrix]] ''P'': i.e.,

:<math> A = DP. </math>

===Group structure===

The [[set (mathematics)|set]] of ''n''&timesthinsp;×&thinsp;''n'' generalized permutation matrices with entries in a [[field (mathematics)|field]] ''F'' forms a [[subgroup]] of the [[general linear group]] GL(''n'', ''F''), in which the group of [[invertible matrix|nonsingular]] diagonal matrices &Delta;Δ(''n'', ''F'') forms a [[normal subgroup]]. Indeed, over all fields except [[GF(2)]], the generalized permutation matrices are the [[normalizer]] of the diagonal matrices, meaning that the generalized permutation matrices are the ''largest'' subgroup of GL(''n'', ''F'') in which diagonal matrices are normal.

The abstract group of generalized permutation matrices is the [[wreath product]] of ''F''^&times;× and ''S''_''n''. Concretely, this means that it is the [[semidirect product]] of &Delta;Δ(''n'', ''F'') by the [[symmetric group]] ''S''_''n'':

:&Delta;(''nS'', _''Fn'') ⋉ {{unicode|&#x22C9Delta;}} (''Sn''_{, ''nF''}),

where ''S''_''n'' acts by permuting coordinates and the diagonal matrices &Delta;Δ(''n'', ''F'') are [[group isomorphism|isomorphic]] to the ''n''-fold product (''F''^&times;×)^''n''.

To be precise, the generalized permutation matrices are a (faithful) [[linear representation]] of this abstract wreath product: a realization of the abstract group as a subgroup of matrices.

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* The subgroup where all entries are ±1 is the [[signed permutation matrices]], which is the [[hyperoctahedral group]].

* The subgroup where the entries are ''m''th [[roots of unity]] <math>\mu_m</math> is isomorphic to a [[generalized symmetric group]].

* The subgroup of diagonal matrices is [[abelian group|abelian]], normal, and a maximal abelian subgroup. The [[quotient group]] is the symmetric group, and this construction is in fact the [[Weyl group]] of the general linear group: the diagonal matrices are a [[maximal torus]] in the general linear group (and are their own [[centralizer]]), the generalized permutation matrices are the normalizer of this torus, and the quotient, <math>N(T)/Z(T) = N(T)/T \cong S_n</math> is the Weyl group.

== Properties ==

* If a nonsingular matrix and its inverse are both [[nonnegative matrices]] (i.e. matrices with nonnegative entries), then the matrix is a generalized permutation matrix.

* The determinant of a generalized permutation matrix is given by <math display="block">\det(G)=\det(P)\cdot \det(D)=\operatorname{sgn}(\pi)\cdot d_{11}\cdot \ldots \cdot d_{nn},</math> where <math>\operatorname{sgn}(\pi)</math> is the [[sign of a permutation|sign]] of the [[permutation]] <math>\pi</math> associated with <math>P</math> and <math>d_{11},\ldots ,d_{nn}</math> are the diagonal elements of <math>D</math>.

== Generalizations ==

One can generalize further by allowing the entries to lie in a [[ring (mathematics)|ring]], rather than in a field. In that case if the non-zero entries are required to be [[unit (ring theory)|units]] in the ring (invertible), one again obtains a group. On the other hand, if the non-zero entries are only required to be non-zero, but not necessarily invertible, this set of matrices forms a [[semigroup]] instead.

One may also schematically allow the non-zero entries to lie in a group ''G,'' with the understanding that matrix multiplication will only involve multiplying a single pair of group elements, not "adding" group elements. This is an [[abuse of notation]], since element of matrices being multiplied must allow multiplication and addition, but is suggestive notion for the (formally correct) abstract group <math>G \wr S_n</math> (the wreath product of the group ''G'' by the symmetric group).

==Signed permutation group==

{{mainfurther|Hyperoctahedral group}}

A '''signed permutation matrix''' is a generalized permutation matrix whose nonzero entries are &plusmn;±1, and are the [[integer]] generalized permutation matrices with integer inverse.

===Properties===

* It is the [[Coxeter group]] <math>B_n</math>, and has [[order (group theory)|order]] <math>2^nnn n!</math>.

* It is the symmetry group of the [[hypercube]] and (dually) of the [[cross-polytope]].

* Its [[index of a subgroup|index]] 2 subgroup of matrices with determinant 1equal to their underlying (unsigned) permutation is the Coxeter group <math>D_n</math> and is the symmetry group of the [[demihypercube]].

* It is a subgroup of the [[orthogonal group]].

==Applications==

===Monomial representations===

{{main|Monomial representation}}

Monomial matrices occur in [[representation theory]] in the context of [[monomial representation]]s. A monomial representation of a group ''G'' is a linear representation {{nowrap|''&rho;ρ'' : ''G'' &rarr;→ GL(''n'', ''F'')}} of ''G'' (here ''F'' is the defining field of the representation) such that the [[image (mathematics)|image]] ''&rho;ρ''(''G'') is a subgroup of the group of monomial matrices.

==References==

* {{cite book | last=Joyner | first=David | title=Adventures in group theory. Rubik's cube, Merlin's machine, and other mathematical toys | edition=2nd updated and revised | ___location=Baltimore, MD | publisher=Johns Hopkins University Press | year=2008 | isbn=978-0-8018-9012-3 | zbl=1221.00013 }}

{{Matrix classes}}

[[Category:Matrices (mathematics)]]

[[Category:Permutations]]

[[Category:Sparse matrices]]