Cyclic sum
A cyclic sum is a summation that cycles through all the values of a function and takes their sum, so to speak.
Rigorous definition
Consider a function 
. The cyclic sum 
is equal to
![\[f(a_1, a_2, \ldots, a_n) + f(a_2, a_3, \ldots, a_n, a_1) + f(a_3, a_4, \ldots, a_n, a_1, a_2) + \ldots + f(a_n, a_1, a_2, \ldots, a_{n-1}).\]](http://latex.artofproblemsolving.com/e/9/f/e9f2db4d8c2f1ebf235f73f55367ac9eb256a8e2.png)
Note that not all permutations of the variables are used; they are just cycled through.
Notation
A cyclic sum is often specified by having the variables to cycle through underneath the sigma, as follows: 
. Note that a cyclic sum need not cycle through all of the variables.
A cyclic sum is also sometimes specified by 
. This notation implies that all variables are cycled through.
