Download Algebraic Methods in the Global Theory of Complex Spaces by Constantin Banica, Octavian Stanasila PDF

By Constantin Banica, Octavian Stanasila

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Extra resources for Algebraic Methods in the Global Theory of Complex Spaces

Sample text

We recall that in a Banach algebra the spectrum a(a) of an element a consists of complex numbers ,\ at which ,\ - a has no inverse and that the spectral radius p(a) = sup{I,\11 >. E a(a)}. 1 the Banach space X is a Banach algebra, then also the functions Z t--+ p(f(z)) and Z t--+ log p(f(z)) are subharmonic. Since the maxi:num of two subharmonic functions is again subharmonic, and log+ u = max{logu, O} under the assumptions above, log+ 11/11 and log+ p(f) are also subharmonic. Subharmonic functions satisfy a maximum principle.

O"k(C). 1. Let Aj = Aj{A) denote the eigenvalues of A, O'j = O'j{A) singular values and recall that we number them in the order of decreasing absolute values. 5 (H. 7) for k = 1,2, ... ,d, with equality for k = d. Proof Let A =diag (AI, ... ,Ad)' By the Schur Decomposition Theorem there exists a unitary U and a strictly upper triangular N such that A = U{A + N)U*. Let Uk E Md,k denote the k first columns of U. Then we have A+N=U*AU= (Uk:Uk ~) with some matrices E, F, G. Since A + N is upper triangular, F = 0 and Uk AUk is upper triangular.

When looking at meromorphic functions F: z 1-+ F(z) E Md we need to be able to do the same thing. 2 For A E Md, put d s(A) := L log+ uj(A). j=1 We may call it the total logarithmic size of A. In order to study simple properties of s(A) we need the following simple technical tool. 3 Let a1 ~ a2 ~ ... such that for k = 1,2, ... ,d we have ~ ad ~ k k j=1 j=1 0, {31 ~ {32 ~ ... ~ {3d ~ 0 be given II aj ::; II (3j. Then d d Llog+(aj)::; Llog+({3j). 10) holds. Otherwise, if we put ad+1 := 0, then let 1::; m ::; d be such that am ~ 1 but a m+1 < 1.