We are concerned with the problem
minp Î Pn maxz Î [ - 1,1] |w(z)(fa (z) - p(z))|,a Î C/[ - 1,1],n = 0 ¼\mathop {min}\limits_{p \in P_n } \mathop {max}\limits_{z \in [ - 1,1]} |w(z)(f_a (z) - p(z))|,a \in C/[ - 1,1],n = 0 \cdots
(1)
of best polynomial approximation of degree n to fa(z)=(z−a)−1 on the unit interval. Here Pn denotes the class of complex polynomials of degree at most n, and ω
... [Show full abstract] belongs to a certain classical family of weight functions.
For real a the solution of this approximation problem is known. In this paper, we obtain the best approximations for purely
imaginary a. For general a, close approximations to the optimal polynomials are derived by solving the approximation problem
expli citly for a certain subclass of Pn. We then use these polynomials to devise an iterative method for the solution of linear systems Ax=b with coefficient matrices
of the form A=cI+dT where T=TH and c, d ∈C. Finally, as a further appication of our results, we derive bounds for the decay rates of the inverses of banded matrices
A=cI+dT.