Finally, we have, corresponding to [2, Theorem 31, the following result, in which the last condition, (3.6), is considerably more complicated than the corresponding condition in [2, Theorem 31.
THEOREM 3. Let S be an n x n complex matrix. Then the following six statements are equivalent:
(3.1) S E P3 (i.e., S is a product of three positive definite hermitian matrices) ;
(3.2) S is conjunctive with an element of .Y3;
(3.3) S is conjunctive with an element of g2;
(3.4) S is conjunctive with a lower triangular matrix of positive diagonal;
(3.5) S is conjunctive with a matrix all of whose leading principal minors aye positive;
(3.6) at least one of the following, (3.6a) or (3.6b), holds.
(3.6a) S has positive determinant and eeieS + eieS* is indefinite for every real 0 ;
(3.6b) the (complex) integral in a E(S) zz Im i tr{ [(l - t)l + tS]-l(S - I)} dt
(where “Im” means “imaginary part” and “tr” means “trace”) exists in the Riemann sense (i.e., all the real eigenvalues of S aye positive) and E(S) itself is zero, and there is an n x 1 complex matrix X such that X*SX = 1. (Note: another condition, (3.6’), equivalent to(3.6) is given at the end of Section 3.)
Proof. The proof that the first five conditions, (3.1) through (3.5), are equivalent to each other is a routine paraphrase of the proof of the corresponding part of [2, Theorem 31. The proof that (3.6) is equivalent to (3.4) will occupy the next four sections of the present paper.
3. GEOMETRIC CONSIDERATIONS
In order to avoid the repeated use of cumbersome expressions in later sections, we introduce in this section some ad hoc terminology (as well as some standard terminology) and derive some basic properties associated with it.