Hermitian matrix - wikipedia Hermitian matrix For matrices with symmetry over the real number field, see symmetric matrix. In mathematics, a Hermitian matrix ( or self - adjoint matrix ) is a complex square matrix that is equal to its own conjugate transpose -- that is, the element in the i - th row and j - th column is equal to the complex conjugate of the element in the j - th row and i - th column, for all indices i and j : a i j = a j i  ̄ ( displaystyle a_ ( ij ) = ( overline ( a_ ( ji ) ) ) ) or A = A T  ̄ ( displaystyle A = ( overline ( A ^ ( \text ( T ) ) ) ) ), in matrix form. Hermitian matrices can be understood as the complex extension of real symmetric matrices. If the conjugate transpose of a matrix A ( displaystyle A ) is denoted by A H ( displaystyle A ^ ( \text ( H ) ) ), then the Hermitian property can be written concisely as A = A H. ( displaystyle A = A ^ ( \text ( H ) ). ) Hermitian matrices are named after Charles Hermite, who demonstrated in 1855 that matrices of this form share a property with real symmetric matrices of always having real eigenvalues. Other, equivalent notations in common use are A H = A † = A ∗ ( displaystyle A ^ ( \text ( H ) ) = A ^ ( dagger ) = A ^ ( ast ) ), although note that in quantum mechanics, A ∗ ( displaystyle A ^ ( ast ) ) typically means the complex conjugate only, and not the conjugate transpose. Contents 1 Applications 2 Examples 3 Properties 4 Further properties 5 Rayleigh quotient 6 See also 7 References 8 External links Applications Hermitian matrices are fundamental to the quantum theory of matrix mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925. Examples In this section, the conjugate transpose of matrix A ( displaystyle A ) is denoted as A H ( displaystyle A ^ ( \text ( H ) ) ), the transpose of matrix A ( displaystyle A ) is denoted as A T ( displaystyle A ^ ( \text ( T ) ) ) and conjugate of matrix A ( displaystyle A ) is denoted as A  ̄ ( displaystyle ( overline ( A ) ) ). See the following example : ( 2 2 + i 4 2 − i 3 i 4 − i 1 ) ( displaystyle ( \begin ( bmatrix ) 2&2 + i&4 2 - i&3&i 4& - i&1 end ( bmatrix ) ) ) The diagonal elements must be real, as they must be their own complex conjugate. Well - known families of Pauli matrices, Gell - Mann matrices and their generalizations are Hermitian. In theoretical physics such Hermitian matrices are often multiplied by imaginary coefficients, which results in skew - Hermitian matrices ( see below ). Here we offer another useful Hermitian matrix using an abstract example. If a square matrix A ( displaystyle A ) equals the multiplication of a matrix and its conjugate transpose, that is, A = B B H ( displaystyle A = BB ^ ( \text ( H ) ) ), then A ( displaystyle A ) is a Hermitian positive semi-definite matrix. Furthermore, if B ( displaystyle B ) is row full - rank, then A ( displaystyle A ) is positive definite. Properties This section needs expansion with : Proof of the properties requested. You can help by adding to it. ( February 2018 ) The entries on the main diagonal ( top left to bottom right ) of any Hermitian matrix are real. Proof : By definition of the Hermitian matrix H i j = H j i  ̄ ( displaystyle H_ ( ij ) = ( overline ( H_ ( ji ) ) ) ) so for i = j the above follows. Note that only complex - valued entries on the main diagonal are forbidden ; Hermetian matricies can have complex - valued entries in their off - diagonal elements, as long as diagonally - opposite entries are complex conjugates. A matrix that has only real entries is Hermitian if and only if it is symmetric. A real and symmetric matrix is simply a special case of a Hermitian matrix. Proof : H i j = H j i  ̄ ( displaystyle H_ ( ij ) = ( overline ( H_ ( ji ) ) ) ) by definition. Thus H = H ( matrix symmetry ) if and only if H i j = H i j  ̄ ( displaystyle H_ ( ij ) = ( overline ( H_ ( ij ) ) ) ) ( H is real ). Every Hermitian matrix is a normal matrix. That is to say, AA = A A. Proof : A = A, so AA = AA = A A. The finite - dimensional spectral theorem says that any Hermitian matrix can be diagonalized by a unitary matrix, and that the resulting diagonal matrix has only real entries. This implies that all eigenvalues of a Hermitian matrix A with dimension n are real, and that A has n linearly independent eigenvectors. Moreover, a Hermitian matrix has orthogonal eigenvectors for distinct eigenvalues. Even if there are degenerate eigenvalues, it is always possible to find an orthogonal basis of C consisting of n eigenvectors of A. The sum of any two Hermitian matrices is Hermitian. Proof : ( A + B ) i j = A i j + B i j = A j i  ̄ + B j i  ̄ = ( A + B ) j i  ̄ ( displaystyle ( A + B ) _ ( ij ) = A_ ( ij ) + B_ ( ij ) = ( overline ( A_ ( ji ) ) ) + ( overline ( B_ ( ji ) ) ) = ( overline ( ( A + B ) _ ( ji ) ) ) ), as claimed. The inverse of an invertible Hermitian matrix is Hermitian as well. The product of two Hermitian matrices A and B is Hermitian if and only if AB = BA. Proof : Note that ( A B ) H = ( A B ) T  ̄ = B T A T  ̄ = B T  ̄ A T  ̄ = B H A H = B A. ( displaystyle ( AB ) ^ ( H ) = ( overline ( ( AB ) ^ ( T ) ) ) = ( overline ( B ^ ( T ) A ^ ( T ) ) ) = ( overline ( B ^ ( T ) ) ) ( overline ( A ^ ( T ) ) ) = B ^ ( H ) A ^ ( H ) = BA. ) Thus ( A B ) H = A B ( displaystyle ( AB ) ^ ( H ) = AB ) if and only if A B = B A ( displaystyle AB = BA ). Thus A is Hermitian if A is Hermitian and n is an integer. For an arbitrary complex valued vector v the product v H A v ( displaystyle v ^ ( \text ( H ) ) Av ) is real because of v H A v = ( v H A v ) H ( displaystyle v ^ ( \text ( H ) ) Av = ( v ^ ( \text ( H ) ) Av ) ^ ( \text ( H ) ) ). This is especially important in quantum physics where Hermitian matrices are operators that measure properties of a system e.g. total spin which have to be real. The Hermitian complex n - by - n matrices do not form a vector space over the complex numbers, C, since the identity matrix I is Hermitian, but i I is not. However the complex Hermitian matrices do form a vector space over the real numbers R. In the 2n - dimensional vector space of complex n × n matrices over R, the complex Hermitian matrices form a subspace of dimension n. If E denotes the n - by - n matrix with a 1 in the j, k position and zeros elsewhere, a basis ( orthonormal w.r.t. the Frobenius inner product ) can be described as follows : E j j ( displaystyle ; E_ ( jj ) ) for 1 ≤ j ≤ n ( displaystyle 1 leq j leq n ) ( n matrices ) together with the set of matrices of the form 1 2 ( E j k + E k j ) ( displaystyle ; ( \frac ( 1 ) ( sqrt ( 2 ) ) ) ( E_ ( jk ) + E_ ( kj ) ) ) for 1 ≤ j < k ≤ n ( displaystyle 1 leq j < k leq n ) ( n − n / 2 matrices ) and the matrices i 2 ( ( E j k − E k j ) ( displaystyle ; ( \frac ( i ) ( sqrt ( 2 ) ) ) ( ( E_ ( jk ) - E_ ( kj ) ) ) for 1 ≤ j < k ≤ n ( displaystyle 1 leq j < k leq n ) ( n − n / 2 matrices ) where i ( displaystyle i ) denotes the complex number − 1 ( displaystyle ( sqrt ( - 1 ) ) ), called the `` imaginary unit ''. If n orthonormal eigenvectors u 1,..., u n ( displaystyle u_ ( 1 ), dots, u_ ( n ) ) of a Hermitian matrix are chosen and written as the columns of the matrix U, then one eigendecomposition of A is A = U Λ U H ( displaystyle A = U Lambda U ^ ( \text ( H ) ) ) where U U H = I = U H U ( displaystyle UU ^ ( \text ( H ) ) = I = U ^ ( \text ( H ) ) U ) and therefore A = ∑ j λ j u j u j H ( displaystyle A = sum _ ( j ) lambda _ ( j ) u_ ( j ) u_ ( j ) ^ ( \text ( H ) ) ), where λ j ( displaystyle lambda _ ( j ) ) are the eigenvalues on the diagonal of the diagonal matrix Λ ( displaystyle ; Lambda ). Further properties Additional facts related to Hermitian matrices include : The sum of a square matrix and its conjugate transpose ( A + A H ) ( displaystyle ( A + A ^ ( \text ( H ) ) ) ) is Hermitian. The difference of a square matrix and its conjugate transpose ( A − A H ) ( displaystyle ( A-A ^ ( \text ( H ) ) ) ) is skew - Hermitian ( also called antihermitian ). This implies that commutator of two Hermitian matrices is skew - Hermitian. An arbitrary square matrix C can be written as the sum of a Hermitian matrix A and a skew - Hermitian matrix B : C = A + B with A = 1 2 ( C + C H ) and B = 1 2 ( C − C H ) ( displaystyle C = A + B quad ( mbox ( with ) ) quad A = ( \frac ( 1 ) ( 2 ) ) ( C + C ^ ( \text ( H ) ) ) quad ( mbox ( and ) ) quad B = ( \frac ( 1 ) ( 2 ) ) ( C-C ^ ( \text ( H ) ) ) ) The determinant of a Hermitian matrix is real : Proof : det ( A ) = det ( A T ) ⇒ det ( A H ) = det ( A )  ̄ ( displaystyle det ( A ) = det ( A ^ ( mathrm ( T ) ) ) quad Rightarrow quad det ( A ^ ( \text ( H ) ) ) = ( overline ( det ( A ) ) ) ) Therefore if A = A H ⇒ det ( A ) = det ( A )  ̄ ( displaystyle A = A ^ ( \text ( H ) ) quad Rightarrow quad det ( A ) = ( overline ( det ( A ) ) ) ). ( Alternatively, the determinant is the product of the matrix 's eigenvalues, and as mentioned before, the eigenvalues of a Hermitian matrix are real. ) Rayleigh quotient Main article : Rayleigh quotient In mathematics, for a given complex Hermitian matrix M and nonzero vector x, the Rayleigh quotient R ( M, x ) ( displaystyle R ( M, x ) ), is defined as : R ( M, x ) : = x H M x x H x ( displaystyle R ( M, x ) : = ( x ^ ( \text ( H ) ) Mx over x ^ ( \text ( H ) ) x ) ). For real matrices and vectors, the condition of being Hermitian reduces to that of being symmetric, and the conjugate transpose x H ( displaystyle x ^ ( \text ( H ) ) ) to the usual transpose x T ( displaystyle x ^ ( \text ( T ) ) ). Note that R ( M, c x ) = R ( M, x ) ( displaystyle R ( M, cx ) = R ( M, x ) ) for any non-zero real scalar c ( displaystyle c ). Also, recall that a Hermitian ( or real symmetric ) matrix has real eigenvalues. It can be shown that, for a given matrix, the Rayleigh quotient reaches its minimum value λ min ( displaystyle lambda _ ( min ) ) ( the smallest eigenvalue of M ) when x ( displaystyle x ) is v min ( displaystyle v_ ( min ) ) ( the corresponding eigenvector ). Similarly, R ( M, x ) ≤ λ max ( displaystyle R ( M, x ) leq lambda _ ( max ) ) and R ( M, v max ) = λ max ( displaystyle R ( M, v_ ( max ) ) = lambda _ ( max ) ). The Rayleigh quotient is used in the min - max theorem to get exact values of all eigenvalues. It is also used in eigenvalue algorithms to obtain an eigenvalue approximation from an eigenvector approximation. Specifically, this is the basis for Rayleigh quotient iteration. The range of the Rayleigh quotient ( for matrix that is not necessarily Hermitian ) is called a numerical range ( or spectrum in functional analysis ). When the matrix is Hermitian, the numerical range is equal to the spectral norm. Still in functional analysis, λ max ( displaystyle lambda _ ( max ) ) is known as the spectral radius. In the context of C * - algebras or algebraic quantum mechanics, the function that to M associates the Rayleigh quotient R ( M, x ) for a fixed x and M varying through the algebra would be referred to as `` vector state '' of the algebra. See also Skew - Hermitian matrix ( anti-Hermitian matrix ) Haynsworth inertia additivity formula Hermitian form Self - adjoint operator Unitary matrix References Jump up ^ Frankel, Theodore ( 2004 ). The Geometry of Physics : an introduction. Cambridge University Press. p. 652. ISBN 0 - 521 - 53927 - 7. Jump up ^ Physics 125 Course Notes at California Institute of Technology Jump up ^ Also known as the Rayleigh -- Ritz ratio ; named after Walther Ritz and Lord Rayleigh. Jump up ^ Horn, R.A. and C.A. Johnson. 1985. Matrix Analysis. Cambridge University Press. pp. 176 -- 180. Jump up ^ Parlet B.N. The symmetric eigenvalue problem, SIAM, Classics in Applied Mathematics, 1998 External links Hazewinkel, Michiel, ed. ( 2001 ) ( 1994 ), `` Hermitian matrix '', Encyclopedia of Mathematics, Springer Science + Business Media B.V. / Kluwer Academic Publishers, ISBN 978 - 1 - 55608 - 010 - 4 Visualizing Hermitian Matrix as An Ellipse with Dr. Geo, by Chao - Kuei Hung from Chaoyang University, gives a more geometric explanation. `` Hermitian Matrices ''. MathPages.com. Retrieved from `` https://en.wikipedia.org/w/index.php?title=Hermitian_matrix&oldid=852436621 '' Categories : Matrices Eponymous scientific concepts Hidden categories : Articles to be expanded from February 2018 All articles to be expanded Talk Contents About Wikipedia বাংলা Беларуская Català Deutsch Eesti Español Esperanto فارسی Français 한국어 Italiano Latviešu Lietuvių Magyar Nederlands 日本 語 Polski Português Русский Slovenščina Српски / srpski Suomi Svenska தமிழ் ไทย Türkçe Українська 中文 20 more Edit links This page was last edited on 29 July 2018, at 00 : 29 ( UTC ). Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: the diagonal elements of a harmitian matrix are
Answer: real