Schatten norm

Let ${\displaystyle H_{1}}$, ${\displaystyle H_{2}}$ be Hilbert spaces, and ${\displaystyle T}$ a (linear) bounded operator from ${\displaystyle H_{1}}$ to ${\displaystyle H_{2}}$. For ${\displaystyle p\in [1,\infty )}$, define the Schatten p-norm of ${\displaystyle T}$ as

${\displaystyle \|T\|_{p}=\mathrm {Tr} (|T|^{p})^{\frac {1}{p}}.}$

If ${\displaystyle T}$ is compact and ${\displaystyle H_{1},\,H_{2}}$ are separable, then

${\displaystyle \|T\|_{p}:={\bigg (}\sum _{n\geq 1}s_{n}^{p}(T){\bigg )}^{1/p}}$

for ${\displaystyle s_{1}(T)\geq s_{2}(T)\geq \cdots s_{n}(T)\geq \cdots \geq 0}$ the singular values of ${\displaystyle T}$, i.e. the eigenvalues of the Hermitian operator ${\displaystyle |T|:={\sqrt {(T^{*}T)}}}$.

In the following we formally extend the range of ${\displaystyle p}$ to ${\displaystyle [1,\infty ]}$ with the convention that ${\displaystyle \|\cdot \|_{\infty }}$ is the operator norm. The dual index to ${\displaystyle p=\infty }$ is then ${\displaystyle q=1}$.

• The Schatten norms are unitarily invariant: for unitary operators ${\displaystyle U}$ and ${\displaystyle V}$ and ${\displaystyle p\in [1,\infty ]}$,

${\displaystyle \|UTV\|_{p}=\|T\|_{p}.}$

• They satisfy Hölder's inequality: for all ${\displaystyle p\in [1,\infty ]}$ and ${\displaystyle q}$ such that ${\displaystyle {\frac {1}{p}}+{\frac {1}{q}}=1}$, and operators ${\displaystyle S\in {\mathcal {L}}(H_{2},H_{3}),T\in {\mathcal {L}}(H_{1},H_{2})}$ defined between Hilbert spaces ${\displaystyle H_{1},H_{2},}$ and ${\displaystyle H_{3}}$ respectively,

${\displaystyle \|ST\|_{1}\leq \|S\|_{p}\|T\|_{q}.}$

(For matrices this can be generalized to ${\displaystyle \|ST\|_{r}\leq \|S\|_{p}\|T\|_{q}}$ for ${\displaystyle {\tfrac {1}{r}}={\tfrac {1}{p}}+{\tfrac {1}{q}}}$.[1])

• Sub-multiplicativity: For all ${\displaystyle p\in [1,\infty ]}$ and operators ${\displaystyle S\in {\mathcal {L}}(H_{2},H_{3}),T\in {\mathcal {L}}(H_{1},H_{2})}$ defined between Hilbert spaces ${\displaystyle H_{1},H_{2},}$ and ${\displaystyle H_{3}}$ respectively,

${\displaystyle \|ST\|_{p}\leq \|S\|_{p}\|T\|_{p}.}$

• Monotonicity: For ${\displaystyle 1\leq p\leq p'\leq \infty }$,

${\displaystyle \|T\|_{1}\geq \|T\|_{p}\geq \|T\|_{p'}\geq \|T\|_{\infty }.}$

• Duality: Let ${\displaystyle H_{1},H_{2}}$ be finite-dimensional Hilbert spaces, ${\displaystyle p\in [1,\infty ]}$ and ${\displaystyle q}$ such that ${\displaystyle {\frac {1}{p}}+{\frac {1}{q}}=1}$, then

${\displaystyle \|S\|_{p}=\sup \lbrace |\langle S,T\rangle |\mid \|T\|_{q}=1\rbrace ,}$

where ${\displaystyle \langle S,T\rangle =\mathrm {tr} (S^{*}T)}$ denotes the Hilbert–Schmidt inner product.

Notice that ${\displaystyle \|\cdot \|_{2}}$ is the Hilbert–Schmidt norm (see Hilbert–Schmidt operator), ${\displaystyle \|\cdot \|_{1}}$ is the trace class norm (see trace class), and ${\displaystyle \|\cdot \|_{\infty }}$ is the operator norm (see operator norm).

For ${\displaystyle p\in (0,1)}$ the function ${\displaystyle \|\cdot \|_{p}}$ is an example of a quasinorm.

An operator which has a finite Schatten norm is called a Schatten class operator and the space of such operators is denoted by ${\displaystyle S_{p}(H_{1},H_{2})}$. With this norm, ${\displaystyle S_{p}(H_{1},H_{2})}$ is a Banach space, and a Hilbert space for p = 2.

Observe that ${\displaystyle S_{p}(H_{1},H_{2})\subseteq {\mathcal {K}}(H_{1},H_{2})}$, the algebra of compact operators. This follows from the fact that if the sum is finite the spectrum will be finite or countable with the origin as limit point, and hence a compact operator (see compact operator on Hilbert space).

The case p = 1 is often referred to as the nuclear norm (also known as the trace norm, or the Ky Fan 'n'-norm[2])

Matrix Norms

• Rajendra Bhatia, Matrix analysis, Vol. 169. Springer Science & Business Media, 1997.
• John Watrous, Theory of Quantum Information, 2.3 Norms of operators, lecture notes, University of Waterloo, 2011.
• Joachim Weidmann, Linear operators in Hilbert spaces, Vol. 20. Springer, New York, 1980.