Bounded inverse theorem

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In

bounded linear operators on Banach spaces
. It states that a bijective bounded linear operator T from one Banach space to another has bounded inverse T−1. It is equivalent to both the open mapping theorem and the closed graph theorem.

Generalization

Theorem

pseudometrizable topological vector space (TVS) onto a Hausdorff TVS that is a Baire space, then A : XY is a homeomorphism
(and thus an isomorphism of TVSs).

Counterexample

This theorem may not hold for normed spaces that are not complete. For example, consider the space X of

supremum norm
. The map T : X → X defined by

is bounded, linear and invertible, but T−1 is unbounded. This does not contradict the bounded inverse theorem since X is not

complete
, and thus is not a Banach space. To see that it's not complete, consider the sequence of sequences x(n) ∈ X given by

converges as n → ∞ to the sequence x(∞) given by

which has all its terms non-zero, and so does not lie in X.

The completion of X is the space of all sequences that converge to zero, which is a (closed) subspace of the p space(N), which is the space of all bounded sequences. However, in this case, the map T is not onto, and thus not a bijection. To see this, one need simply note that the sequence

is an element of , but is not in the range of .

See also

  • Almost open linear map
     – Map that satisfies a condition similar to that of being an open map.
  • Closed graph
     – Graph of a map closed in the product space
  • Closed graph theorem – Theorem relating continuity to graphs
  • Open mapping theorem (functional analysis) – Condition for a linear operator to be open
  • Surjection of Fréchet spaces – Characterization of surjectivity
  • Webbed space – Space where open mapping and closed graph theorems hold

References

Bibliography

  • OCLC 840293704
    .
  • Narici, Lawrence; Beckenstein, Edward (2011). Topological Vector Spaces. Pure and applied mathematics (Second ed.). Boca Raton, FL: CRC Press. .
  • Renardy, Michael; Rogers, Robert C. (2004). An introduction to partial differential equations. Texts in Applied Mathematics 13 (Second ed.). New York: Springer-Verlag. pp. 356. . (Section 8.2)
  • .