Hyponormal operator

In mathematics, especially operator theory, a hyponormal operator is a generalization of a normal operator. In general, a bounded linear operator T on a complex Hilbert space H is said to be p-hyponormal ($0<p\leq 1$) if:

$(T^{*}T)^{p}\geq (TT^{*})^{p}$

(That is to say, $(T^{*}T)^{p}-(TT^{*})^{p}$ is a positive operator.) If $p=1$, then T is called a hyponormal operator. If $p=1/2$, then T is called a semi-hyponormal operator. Moreover, T is said to be log-hyponormal if it is invertible and

$\log(T^{*}T)\geq \log(TT^{*}).$

An invertible p-hyponormal operator is log-hyponormal. On the other hand, not every log-hyponormal is p-hyponormal.

The class of semi-hyponormal operators was introduced by Xia, and the class of p-hyponormal operators was studied by Aluthge, who used what is today called the Aluthge transformation.

Every subnormal operator (in particular, a normal operator) is hyponormal, and every hyponormal operator is a paranormal convexoid operator. Not every paranormal operator is, however, hyponormal.

References

• Huruya, Tadasi (1997). "A Note on p-Hyponormal Operators". Proceedings of the American Mathematical Society. 125 (12): 3617–3624. doi:10.1090/S0002-9939-97-04004-5. JSTOR 2162263.