Compound matrix: Difference between revisions

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== Definition ==

Let {{math|''A''}} be an {{math|''m''&thinsp;×&thinsp;''n''}} matrix with [[real number|real]] or [[complex number|complex]] entries.{{efn|The definition, and the purely algebraic part of the theory, of compound matrices requires only that the matrix have entries in a [[commutative ring]]. In this case, the matrix corresponds to a [[module homomorphism|homomorphism]] of [[finitely generated module|finitely generated]] [[free module]]s.}} If {{math|''I''}} is a [[subset]] of size {{math|''r''}} of {{math|{1, ..., ''m''<nowiki>}</nowiki>}} and {{math|''J''}} is a subset of size {{math|''s''}} of {{math|{1, ..., ''n''<nowiki>}</nowiki>}}, then the '''{{math|(''I'', ''J''&{{hairsp;}})}}-submatrix of {{math|''A''}}''', written {{math|''A''_{''I'', ''J''}}}&{{hairsp;}}, is the submatrix formed from {{math|''A''}} by retaining only those rows indexed by {{math|''I''}} and those columns indexed by {{math|''J''}}. If {{math|1=''r'' = ''s''}}, then {{math|det&thinsp;''A''_{''I'', ''J''}}} is the '''{{math|(''I'', ''J''&{{hairsp;}})}}-[[minor (linear algebra)|minor]]''' of {{math|''A''}}.

The '''''r''&{{hairsp;}}th compound matrix''' of {{math|''A''}} is a matrix, denoted {{math|''C''_{''r''&thinsp;}(''A'')}}, is defined as follows. If {{math|''r'' > min(''m'', ''n'')}}, then {{math|''C''_{''r''&thinsp;}(''A'')}} is the unique {{math|0&thinsp;×&thinsp;0}} matrix. Otherwise, {{math|''C''_{''r''&thinsp;}(''A'')}} has size <math display="inline">\binom{m}{r} \!\times\! \binom{n}{r}</math>. Its rows and columns are indexed by {{math|''r''}}-element subsets of {{math|{1, ..., ''m''<nowiki>}</nowiki>}} and {{math|{1, ..., ''n''<nowiki>}</nowiki>}}, respectively, in their [[lexicographic order]]. The entry corresponding to subsets {{math|''I''}} and {{math|''J''}} is the minor {{math|det&thinsp;''A''_{''I'', ''J''}}}.

In some applications of compound matrices, the precise ordering of the rows and columns is unimportant. For this reason, some authors do not specify how the rows and columns are to be ordered.<ref>Kung, Rota, and Yan, p. 305.</ref>

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For example, consider the matrix

:<math>A = \begin{pmatrix} 1 & 2 & 3 & 4 \\ 5 & 6 & 7 & 8 \\ 9 & 10 & 11 & 12 \end{pmatrix}.</math>

The rows are indexed by {{math|{1, 2, 3<nowiki>}</nowiki>}} and the columns by {{math|{1, 2, 3, 4<nowiki>}</nowiki>}}. Therefore, the rows of {{math|''C''_{2&{{hairsp;}}}(''A'')}} are indexed by the sets

:<math>\{1, 2\} < \{1, 3\} < \{2, 3\}</math>

and the columns are indexed by

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Let {{math|''c''}} be a scalar, {{math|''A''}} be an {{math|''m''&thinsp;×&thinsp;''n''}} matrix, and {{math|''B''}} be an {{math|''n''&thinsp;×&thinsp;''p''}} matrix. For {{math|''k''}} a positive [[integer]], let {{math|''I''_''k''}} denote the {{math|''k''&thinsp;×&thinsp;''k''}} [[identity matrix]]. The [[transpose]] of a matrix {{math|''M''}} will be written {{math|''M''{{i sup|T}}}}, and the [[conjugate transpose]] by {{math|''M''{{i sup|*}}}}. Then:<ref>Horn and Johnson, p. 22.</ref>

* {{math|1=''C''_{0&{{hairsp;}}}(''A'') = ''I''₁}}, a {{math|1&thinsp;×&thinsp;1}} identity matrix.

* {{math|1=''C''₁(''A'') = ''A''}}.

* {{math|1=''C''_{''r''&thinsp;}(''cA'') = ''c''{{i sup|''r''}}''C''_{''r''&thinsp;}(''A'')}}.

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* If {{math|1&thinsp;≤ ''r'' ≤ min(''m'', ''n'')}}, then {{math|1=''C''_{''r''&thinsp;}(''A''{{i sup|T}}) = ''C''_{''r''&thinsp;}(''A''){{i sup|T}}}}.

* If {{math|1&thinsp;≤ ''r'' ≤ min(''m'', ''n'')}}, then {{math|1=''C''_{''r''&thinsp;}(''A''^*) = ''C''_{''r''&thinsp;}(''A'')^*}}.

* {{math|1=''C''_{''r''&thinsp;}(''AB'') = ''C''_{''r''&thinsp;}(''A'')&{{hairsp;}}''C''_{''r''&thinsp;}(''B'')}}, which is closely related to [[Cauchy–Binet formula]].

Assume in addition that {{math|''A''}} is a [[square matrix]] of size {{math|''n''}}. Then:<ref>Horn and Johnson, pp. 22, 93, 147, 233.</ref>

* {{math|1=''C''_{''n''&{{hairsp;}}}(''A'') = det&thinsp;''A''}}.

* If {{math|''A''}} has one of the following properties, then so does {{math|''C''_{''r''&thinsp;}(''A'')}}:

** [[Upper triangular]],

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{{see also|Exterior algebra}}

Give {{math|'''R'''^''n''}} the [[canonical basis|standard coordinate basis]] {{math|'''e'''₁, ..., '''e'''_''n''}}. The {{math|''r''}}&{{hairsp;}}th exterior power of {{math|'''R'''^''n''}} is the [[vector space]]

:<math>\wedge^r \mathbf{R}^n</math>

whose [[basis (linear algebra)|basis]] consists of the formal symbols

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Suppose that {{math|''A''}} is an {{math|''m''&thinsp;×&thinsp;''n''}} matrix. Then {{math|''A''}} corresponds to a [[linear transformation]]

:<math>A \colon \mathbf{R}^n \to \mathbf{R}^m.</math>

Taking the {{math|''r''}}&{{hairsp;}}th exterior power of this linear transformation determines a linear transformation

:<math>\wedge^r A \colon \wedge^r \mathbf{R}^n \to \wedge^r \mathbf{R}^m.</math>

The matrix corresponding to this linear transformation (with respect to the above bases of the exterior powers) is {{math|''C''_{''r''&thinsp;}(''A'')}}. Taking exterior powers is a [[functor]], which means that<ref>Joseph P.S. Kung, Gian-Carlo Rota, and [[Catherine Yan|Catherine H. Yan]], ''[[Combinatorics: The Rota Way]]'', Cambridge University Press, 2009, p. 306. {{isbn|9780521883894}}</ref>

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{{see also|Adjugate matrix}}

Let {{math|''A''}} be an {{math|''n''&thinsp;×&thinsp;''n''}} matrix. Recall that its '''{{mvar|r}}&{{hairsp;}}th higher adjugate matrix''' {{math|adj_{''r''&{{hairsp;}}}(''A'')}} is the <math display="inline">\binom{n}{r} \!\times\! \binom{n}{r}</math> matrix whose {{math|(''I'', ''J''&{{hairsp;}})}} entry is

:<math>(-1)^{\sigma(I) + \sigma(J)} \det A_{J^c, I^c},</math>

where, for any set {{math|''K''}} of integers, {{math|''σ''(''K'')}} is the sum of the elements of {{math|''K''}}. The '''adjugate''' of {{math|''A''}} is its 1st higher adjugate and is denoted {{math|adj(''A'')}}. The generalized [[Laplace expansion]] formula implies

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and let {{math|''J''}} denote the ''[[exchange matrix]]'':

:<math>J = \begin{pmatrix} & & 1 \\ & \cdots & \\ 1 & & \end{pmatrix}.</math>

Then '''Jacobi's theorem''' states that the {{math|''r''}}&{{hairsp;}}th higher adjugate matrix is:<ref name="NambiarSreevalsan2001">{{cite journal|last1=Nambiar|first1=K.K.|last2=Sreevalsan|first2=S.|title=Compound matrices and three celebrated theorems|journal=Mathematical and Computer Modelling|volume=34|issue=3–4|year=2001|pages=251–255|issn=0895-7177|doi=10.1016/S0895-7177(01)00058-9|doi-access=free}}</ref><ref name="Price1947">{{cite journal|last1=Price|first1=G. B.|authorlink=G. B. Price|title=Some Identities in the Theory of Determinants|journal=The American Mathematical Monthly|volume=54|issue=2|year=1947|pages=75–90|issn=0002-9890|doi=10.2307/2304856|jstor=2304856}}</ref>

:<math>\operatorname{adj}_r(A) = JC_{n-r}(SAS)^TJ.</math>