We show that the perturbation estimate for the matrix equation X − A * X q A = Q (0 < q < 1), due to G. Jia and D. Cao, published in this journal, is wrong.
Mathematics Subject Classification
PreliminariesThe following definitions and the notations are the same as in [1]. We denote by C n×n the set of n × n complex matrices, and we denote by H n×n the set of n × n Hermitian matrices. We write X ≥ Y (X > Y) if X − Y is positive semidefinite (definite). If X, Y ∈ H n×n such that X ≤ Y , then [X, Y ] will be the set of all Z ∈ H n×n satisfying X ≤ Z ≤ Y . We denote by · the spectral norm. For Hermitian matrix N , let λ max (N ) and λ min (N ) be the maximal and minimal eigenvalue of N , respectively.Consider the matrix equationwhere A, Q ∈ C n×n and Q is positive definite matrix. The existence and uniqueness of its positive definite solution X is proved in [2], furthermore X ∈ [βI, αI ], where