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Compound operator with built-in evaluation rules.
is by default interpreted as Sum[f, i, imax].
is by default interpreted as Sum[f, i, imin, imax].
Not the same as the Greek letter \[CapitalSigma].
See The Mathematica Book: Section 1.10.4, Section 1.10.4, Section 3.10.1, Section 3.10.3 and Section 3.10.4.
See also: \[Product] , \[Integral] .