Be a normed space and a compact operator, . It is , thus steady - - the space of linear bounded operators on .
If the Neumann series converges in space with respect to the operator norm , then is invertible and it holds
.
The Neumann series converges if is a Banach space and holds for the operator norm . Then also applies:
.
Weaker conditions are also known under which the series can converge, e.g. B. it is sufficient if the condition is only valid for a power of the operator . Then
Invertibility of linear operators
Is a Banach space, e.g. B. , and a bounded operator, e.g. B. a square matrix , then for each scaling factor as
With
being represented.
If there is now a scaling factor with which the induced operator norm applies, then it is invertible and the inverse is, using the Neumann series,
.
Openness of the set of invertible operators
Let be two Banach spaces and an invertible operator. Then for every further operator :
If the estimate with applies to the distance in the operator norm from to , then it is also invertible and the inverse has the operator norm
.
As proof: It is broken down and the Neumann series is applied to the second factor. Convergence is assured, because according to the prerequisite:
.
As a result, the set of invertible operators is open with regard to the topology of the operator norm.