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University of Northern Iowa: Summation Notation
This site from the University of Northern Iowa explains the sigma notation and gives an example.
University of California
University of California, Davis: The Calculus Page
This site, from the University of California, provides several problems from beginning differential calculus, as well as beginning integral calculus. The problems can be accessed by clicking on the link, and the information provided is...
Other
The Sigma Notation for Summation of Series
Conprehensive site explaining sigma notation. Site has many formulas and a few examples