Closed range theorem

In the mathematical theory of Banach spaces, the closed range theorem gives necessary and sufficient conditions for a closed densely defined operator to have closed range. The theorem was proved by Stefan Banach in his 1932 Théorie des opérations linéaires.

Let $X$ and $Y$ be Banach spaces, $T\colon D(T)\to Y$ a closed linear operator whose domain $D(T)$ is dense in $X$, and $T'$ the transpose of $T$. The theorem asserts that the following conditions are equivalent:

• $R(T)$, the range of $T$, is closed in $Y$,
• $R(T')$, the range of $T'$, is closed in $X'$, the dual of $X$,
• $R(T)=N(T')^{\perp }=\{y\in Y|\langle x^{*},y\rangle =0\quad {\text{for all}}\quad x^{*}\in N(T')\}$,
• $R(T')=N(T)^{\perp }=\{x^{*}\in X'|\langle x^{*},y\rangle =0\quad {\text{for all}}\quad y\in N(T)\}$.

Several corollaries are immediate from the theorem. For instance, a densely defined closed operator $T$ as above has $R(T)=Y$ if and only if the transpose $T'$ has a continuous inverse. Similarly, $R(T')=X'$ if and only if $T$ has a continuous inverse.

See also

• Fundamental theorem of linear algebra

References

• Yosida, K. (1980), Functional Analysis, Grundlehren der Mathematischen Wissenschaften (Fundamental Principles of Mathematical Sciences), vol. 123 (6th ed.), Berlin, New York: Springer-Verlag.