Difference between revisions of "Completely-continuous operator"
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''Completely-Continuous Operator'' | ''Completely-Continuous Operator'' | ||
− | A bounded linear operator | + | A bounded linear operator $f$, acting from a [[Banach space|Banach space]] $X$ into another space $Y$, that transforms weakly-convergent sequences in $X$ to norm-convergent sequences in $Y$. Equivalently, an operator $f$ is completely-continuous if it maps every relatively weakly compact subset of $X$ into a relatively compact subset of $Y$. It is easy to see that every compact operator is completely continuous, however the converse is false. For example, recall that the Banach space $X=l_1$ has the Schur Property, that is weak sequential and norm sequential convergence coincide. It follows that the identity operator from $X$ to $X$ is completely-continuous, but it is not compact since $X$ is infinite-dimensional. If $X$ is reflexive, then every completely-continuous operator is compact, so the two classes of operators do coincide in that case. The term "completely-continuous operator" originally meant what we now call "compact operator", which has sometimes resulted in confusion. |
− | It can be assumed that the space | + | It can be assumed that the space $X$ is separable (for $Y$ this is not a necessary condition; however, the image of a completely-continuous operator is always separable). |
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====References==== | ====References==== | ||
− | <table><TR><TD valign="top">[1]</TD> <TD valign="top"> D. Hilbert, "Grundzüge einer allgemeinen Theorie der linearen Integralgleichungen" , Chelsea, reprint (1953)</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top"> F. Riesz, "Sur les opérations fonctionelles linéaires" ''C.R. Acad. Sci. Paris Sér. I Math.'' , '''149''' (1909) pp. 974–977</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top"> S.S. Banach, "Théorie des opérations linéaires" , Hafner (1932)</TD></TR></table> | + | <table><TR><TD |
− | + | valign="top">[1]</TD> <TD valign="top"> D. Hilbert, "Grundzüge einer allgemeinen Theorie der linearen Integralgleichungen" , Chelsea, reprint (1953)</TD></TR><TR><TD | |
− | + | valign="top">[2]</TD> <TD valign="top"> F. Riesz, "Sur les opérations fonctionelles linéaires" ''C.R. Acad. Sci. Paris Sér. I Math.'' , '''149''' (1909) pp. 974–977</TD></TR><TR><TD | |
+ | valign="top">[3]</TD> <TD valign="top"> S.S. Banach, "Théorie des opérations linéaires" , Hafner (1932)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top"> R. E. Megginson, "An Introduction to Banach Space Theory" , Springer (1998) pp. 336-339 </TD></TR><TR><TD | ||
+ | valign="top">[5]</TD> <TD valign="top"> A. Pietsch, "History of Banach Spaces and Linear Operators" , Birkhauser (2007) pp. 49-50 </table> | ||
====Comments==== | ====Comments==== |
Latest revision as of 20:40, 2 April 2013
Completely-Continuous Operator
A bounded linear operator $f$, acting from a Banach space $X$ into another space $Y$, that transforms weakly-convergent sequences in $X$ to norm-convergent sequences in $Y$. Equivalently, an operator $f$ is completely-continuous if it maps every relatively weakly compact subset of $X$ into a relatively compact subset of $Y$. It is easy to see that every compact operator is completely continuous, however the converse is false. For example, recall that the Banach space $X=l_1$ has the Schur Property, that is weak sequential and norm sequential convergence coincide. It follows that the identity operator from $X$ to $X$ is completely-continuous, but it is not compact since $X$ is infinite-dimensional. If $X$ is reflexive, then every completely-continuous operator is compact, so the two classes of operators do coincide in that case. The term "completely-continuous operator" originally meant what we now call "compact operator", which has sometimes resulted in confusion.
It can be assumed that the space $X$ is separable (for $Y$ this is not a necessary condition; however, the image of a completely-continuous operator is always separable).
The class of compact operators is the most important class of the set of completely-continuous operators (cf. Compact operator).
References
[1] | D. Hilbert, "Grundzüge einer allgemeinen Theorie der linearen Integralgleichungen" , Chelsea, reprint (1953) |
[2] | F. Riesz, "Sur les opérations fonctionelles linéaires" C.R. Acad. Sci. Paris Sér. I Math. , 149 (1909) pp. 974–977 |
[3] | S.S. Banach, "Théorie des opérations linéaires" , Hafner (1932) |
[4] | R. E. Megginson, "An Introduction to Banach Space Theory" , Springer (1998) pp. 336-339 |
[5] | A. Pietsch, "History of Banach Spaces and Linear Operators" , Birkhauser (2007) pp. 49-50 |
Comments
References
[a1] | N. Dunford, J.T. Schwartz, "Linear operators. General theory" , 1 , Interscience (1958) |
[a2] | A.E. Taylor, D.C. Lay, "Introduction to functional analysis" , Wiley (1980) |
Completely-continuous operator. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Completely-continuous_operator&oldid=29567