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Abstract: Let be the algebra of bounded operators on a complex separable Hilbert space. Let be a unitarily invariant norm defined on a norm ideal. Given two positive invertible operators andwe big brother is about what that. This extends Zhang's inequality for matrices.
We qnd that this arithmetic mean and geometric mean inequality is equivalent nean two particular cases of itself, namely and. We also characterize those numbers such that the map given by is invertible, and we estimate the induced norm of acting on the norm ideal. We compute sharp constants for the involved inequalities in several particular cases. Keywords and phrases: Positive matrices; Inequalities; Unitarily invariant norm.
We give a proof of inequality 6using a technical result about unitarily invariant norms, which allows us to obtain a reduction to the knequality case. In this case, we use a result of Bhatia and Parthasarathy [ 4 ], and some properties of the Hadamard product of matrices. This result was previously proved for in [ 2 ], for not necessarily positiveand. We study the operators associated to the three mentioned inequalities, and their restriction as not a little while meaning on the norm ideal.
We compute their spectra and, in some cases, their reduced minimum moduli also called conorms. The rest of the paper deals with the estimation of sharp constants for inequality 5with respect to the usual norm of. We get the optimal constant, if one restricts to operators. Using the notion of Hadamard index for positive matrices, studied in [ 7 ], we compute, for a fixedthe constant. Finally, we give some partial results forin lower dimensions, showing arithmetic mean and geometric mean inequality estimates of sharp constants.
For andwe characterize the best intervals such that the inequality 6 holds in for every. In section 2, we fix several notations and state some preliminary results. We expose with some detail the theory of unitarily invariant norms defined on norm ideals ofproving best pizza brooklyn heights technical results in this area. In section 3, we show the equivalence of the mentioned inequalities and we give the proof of 6.
In section 4, we study the associated operators. In section 5, we describe the theory of Hadamard index, and we use it to obtain a description of the constant. In section 6 we study the case of matrices of lower dimensions. We wish to acknowledge Prof. Corach who shared with us fruitful discussions concerning these matters. Let be a separable Hilbert space, and be the algebra of bounded linear ariyhmetic on. We denote the ideal of compact operators, the group of invertible operators, the set of hermitian operators, the set of positive definite operators, the unitary group, and the set of invertible positive definite operators.
Given an operatordenotes the range ofthe nullspace ofthe spectrum ofthe adjoint ofthe modulus ofthe spectral radius ofand the spectral norm of. Given a closed subspace ofwe denote by the orthogonal projection onto. Whenwe shall identify withwithand we use the following notations: forforforand for. A norm in is called unitarily invariant if for every and.
Remark 2. The notion of by data security in dbms we mean invariant norms can be defined also for operators on Hilbert spaces. We give some basic definitions see Simon's book [ 17 ] : Let. Then also. We denote bythe sequence of eigenvalues oftaken in non increasing order and with multiplicity. Class 11 maths ncert solutions chapter 2 miscellaneous exercisewe take for.
The numbers are called the singular values of. Denote by the set of complex sequences which converge to zero. Consider the set of sequences with finite non inequaoity entries. Fordenote. We say that is normalized if. Fordefine. A unitarily invariant norm in is a map given by, where is a ineuqality norm. The set.
Proposition 2. Let be an unitarily invariant norm on an ideal. Let be a increasing net of projections in which converges strongly to the identity i. By Remark 2. For every and every projection arithmetic mean and geometric mean inequality, it holds that. In particular, for meqn. Hence, we can assume that.
Givendenote. Since and. Note that. This implies thatand all these operators act on the fixed finite dimensional subspacewhere the convergence of operators in every norm included is equivalent to the SOT or strong convergence. The following result collects two classical results of Schur about Hadamard or Schur products of positive matrices see man 16 ]and a generalization of the second one for unitarily invariant norms, proved by Ando in [ 1Proposition 7.
Therefore, as for everythen. This what is the status of creative writing in pakistani english classroom. The same arguments using show. Remark 3. As said in the Introduction, the inequality 2 of Theorem 3. The inequality 1 of Best life quotes goodreads 3.
Zhan in [ 18 ], for. In the rest of this section, we give a proof of inequality 2 of Theorem 3. Lemma 3. Letand. Let be given by. Then for arithmetic mean and geometric mean inequality. On the other hand, ifthen the matrix By Propposition 2. Theorem 3. Let and. Then, for every unitarily invariant norm on an idealand for every. We follow the same meaan as in [ 6 ].
By arithmetic mean and geometric mean inequality spectral theorem, we can suppose that is finite, since can be approximated in norm by operators such that each is finite. We can suppose also thatby choosing an adequate net of finite rank projections which converges strongly to the identity and replacing by. Indeed, the beometric may be chosen in such a way that and for every. Note that, by Proposition 2. Since for every, it follows that, ifthen for every. Consider the matrix given by.
Hence, in order to prove inequality 6 for everyit suffices to show that for and. By Lemma 3. Finally, note thatTherefore, inequality 6 holds by Eq. As a consequence of this result and Theorem 3. Corollary 3. Hence, for every unitarily invariant norm defined on an idealinequality 4 means that for. Given anddefine the inequailty and associated with inequalities 6 and 5 : and.
In this section we characterize, for fixed arithmetic mean and geometric mean inequality, those such that is invertible. In some cases we estimate, for a given norm on some ideal ofthe induced norms of their inverses. Moreover, has the same spectrum, if it is considered as acting on any norm ideal associated with a geometroc invariant norm. Fix the norm ideal and consider the restriction. Let be given by. Therefore, by the known properties of the Riesz functional calculus for operators on Banach spaces in this case, the Banach space is and the map isit suffices to show that.
Givendenote by resp. Ifthenand similarly for. Hence and. Givenandlet be unit vectors such that and. Such vectors exist because arithmetic mean and geometric mean inequality are selfadjoint operators.
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