Condition for the numerical range to contain an elliptic disc

Abstract

For an n-by-n matrix A and an elliptic disc E in the plane, we show that the sum of the number of common supporting lines and the number of common intersection points to E and the numerical range W (A) of A should be at least 2n + 1 in order to guarantee that E be contained in W(A). This generalizes previous results of Anderson and Thompson. As an application, our result is used to verify a special case of the Poncelet property conjecture. (C) 2003 Elsevier Science Inc. All rights reserved.