We investigate a fundamental question about the discretization of continuous-time controllers. We consider a linear feedback system that works in continuous time, and has satisfactory performance. We want to replace the controller with a combination of a discrete-time system (a digital processor), an analog-to-digital converter (a filter with a sampler) and a digital-to-analog converter (a hold...

Characterisations of those separable C∗-algebras that have type I injective envelopes or W∗-algebra injective envelopes are presented. An operator system I is injective if for every inclusion E ⊂ F of operator systems each completely positive linear map ω : E → I has a completely positive extension to F . An injective envelope of an operator system E is an injective operator system I such that ...

We show that operators on n×n matrices which are representable in the form T (X) = ∑` i=1 aiXbi (for ai and bi n×n matrices) and are k-positive for k = [ √ `] must be completely positive. As a consequence, elementary operators on a C*-algebra with minimal length ` which are k-positive for k = [ √ `] must be completely positive. For A a C*-algebra, an operator T :A→ A is called an elementary ope...