An operator assigns to every function a function . It is therefore a mapping between two Function Spaces. If the range is on the Real Line or in the Complex Plane, the mapping is usually called a Functional instead.

**References**

Gohberg, I.; Lancaster, P.; and Shivakuar, P. N. (Eds.).
*Recent Developments in Operator Theory and Its Applications.* Boston, MA: Birkhäuser, 1996.

Hutson, V. and Pym, J. S. *Applications of Functional Analysis and Operator Theory.* New York: Academic Press, 1980.

© 1996-9

1999-05-26