16 CONWAY, HERRERO, AND MORREL
(b) P(Y) c P(X) and ind {X - Y) = ind {X - X) and min ind {X - Y)k
min ind {X - X)k for all X in P(Y) and all k 1 ;
(c) dim itf(Y; X.) = dim #(Y; X) for all X in o0(Y) ; then Y is in the
closure of the similarity orbit of X ,
5(X) = { WXW"1 : W is invertible in CB(fhQ} .
The next proposition is a variation on a result of [6] (also see page 136
of [20]). Essentially it is a special case of that result that will be needed in
this paper.
1.9 Proposition If T e B(9{) and e 0 , then there is an operator S such
that I I S - Tl I e and S is similar to a direct sum Sx © © S
n
© F , where
these direct summands satisfy the following properties:
(a) F is a finite rank operator with a(F) c J0(T) and for X in a(F),
F | ^(F; X) similar to T | #|T; X) ;
(b) for each j ,
CJ(SJ)
is connected;
(c)
G(F)
n
G(SJ)
= 0 = a(Sj) n afS^ for i * j and 1 i, j n ;
(d) for each j , alre(Sj) is the closure of an analytic Cauchy domain and
a(Sj) \ cilre(Sj) is an analytic Cauchy domain;
(e) P(S) c P(T) , P+(S) has only a finite number of components, and
for each X in P(S) , nul (X - S) = nul {X - T) and nul (X - S)* = nul (X - T)* .
(f) oe(T) c oe(S) , alre(T) c cJlre(S) , and each component of c
e
(S)
contains at least one component of tfe(T) .
Proof. Let 8 0 and let A be an analytic Cauchy domain with alre(T) e A c
[alre(T) ]g and 3A n a0(T) = 0 . Let N be a normal operator with o(N) = cl A .
By Proposition 1.4 of [6] (also see Chapter 3 in [20]) there is an operator S
with I I T - SI I 25 such that S is similar to T © N . (This could be
obtained as a consequence of the Similarity Orbit Theorem, but this would
be putting the cart before the horse.) Consequently
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