Sumclass symbols, or accumulation symbols, are symbols whose sub and superscripts appear directly below and above the symbol rather than beside it. For example, the following example illustrates that \sum
is one of these elite symbols whereas \Sigma
is not. The terminology from AMSLaTeX documentation.
Table of sumclass symbols[edit  edit source]
Using sum[edit  edit source]
LaTeX markup...  ...results in:  ...is used for: 

\sum\limits_{i=1}^n i^2 = \frac{n(n+1)(2n+1)}{6}

the \limits tag puts the limits below and above the sigma symbol. It is typically used in equations  
\sum\nolimits_{P_i \in Paths(I)} Probes(P_{i})

the \nolimits tag puts the limits on the right of the sigma symbol. It is typically used in the math wired in the text 
TeX is smart enough to only show \sum
in its expanded form in the displaymath environment. In the regular math environment, \sum
does the right thing and revert to nonsumclass behavior, thus conserving vertical space.
Using prod[edit  edit source]
Another common sumclass symbol is \prod
. As in \sum
we can use the directive \limits
or \nolimits
in order to show the limits on topdown or on the right.
LaTeX markup...  ...results in:  ...is used for: 

\prod\limits_{i=1}^n x = x^n

the product of a sequence of factors 
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