Operator gauges and their normed carriers #
An operator gauge g : OperatorGauge 𝕜 E F assigns an extended value g A ∈ [0, ∞] to every
bounded operator A : E →L[𝕜] F between normed spaces. It is subadditive, absolutely
homogeneous, and dominates the operator norm. Examples are the Schatten and Ky Fan gauges on
Hilbert spaces, and the nuclear and absolutely summing gauges on Banach spaces; the operator-ideal
families of TauCeti.Analysis.OperatorIdeal.Family restrict to operator gauges on each pair of
Hilbert spaces.
The operators of finite gauge form the submodule g.finiteSubmodule of E →L[𝕜] F. The finite
real value of the gauge is a norm on it, in general finer than the operator norm. Since that norm
differs from the operator norm, the finite-gauge operators are packaged as a separate type
g.Carrier rather than as the subtype of g.finiteSubmodule, which would inherit the operator
norm.
When F is a normed space, the carrier g.Carrier is a normed space over 𝕜 with
‖T‖ = (g T).toReal. No information is lost in passing to the real norm, since ‖T‖ₑ = g T, and
the inclusion into bounded operators is a contraction. If moreover F is complete and the gauge
is lower semicontinuous for the operator-norm topology, then the carrier is a Banach space: a
Cauchy sequence for the gauge norm is Cauchy for the operator norm, and lower semicontinuity
bounds the gauge of the distance to its operator-norm limit.
Main declarations #
TauCeti.OperatorGauge: an extended gauge on bounded operators.TauCeti.OperatorGauge.finiteSubmodule: the submodule of operators of finite gauge.TauCeti.OperatorGauge.Carrier: the normed space of operators of finite gauge.TauCeti.OperatorGauge.Carrier.norm_def,TauCeti.OperatorGauge.Carrier.enorm_eq_gauge: the norm of the carrier is the gauge.TauCeti.OperatorGauge.Carrier.toContinuousLinearMapL: the contractive inclusion of the carrier into bounded operators.TauCeti.OperatorGauge.completeSpace_carrier_of_lowerSemicontinuous: a lower semicontinuous gauge into a Banach space has a complete carrier.
References #
- A. Pietsch, Operator Ideals, North-Holland, 1980, Chapter 6.
- B. Simon, Trace Ideals and Their Applications, second edition, AMS, 2005, Chapter 2.
An operator gauge on E →L[𝕜] F: an extended nonnegative function on bounded operators that
is subadditive, absolutely homogeneous and dominates the operator norm.
The gauge of a bounded operator.
The gauge is subadditive.
The gauge is absolutely homogeneous.
The gauge dominates the extended operator norm.
Instances For
An operator gauge is subadditive.
An operator gauge is absolutely homogeneous.
An operator gauge dominates the extended operator norm.
An operator gauge into a normed space vanishes exactly at zero.
The submodule of operators of finite gauge #
The operators of finite gauge form a submodule of the bounded operators.
Instances For
The normed carrier #
The operators of finite gauge, normed by the finite real value of the gauge.
This is a separate type from the subtype of g.finiteSubmodule, which carries the operator
norm.
The underlying bounded operator.
The underlying operator has finite gauge.
Instances For
The underlying operators of the carrier are exactly the operators of finite gauge.
The gauge of the underlying operator, as an additive group norm on the carrier.
Instances For
The norm of an element of the carrier is the finite real value of its gauge.
The extended norm of an element of the carrier is its gauge.
Passing from the gauge to the real norm of the carrier loses no information.
The operator norm is dominated by the norm of the carrier.
The inclusion of the carrier into the bounded operators, as a bounded linear map.
Instances For
The inclusion of the carrier into the bounded operators is a contraction.
Completeness #
If the target is complete and the gauge is lower semicontinuous for the operator-norm topology, then the carrier is complete.