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Mathematica Teacher's Edition

Built-in Functions

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SameQ

lhs === rhs yields True if the expression lhs is identical to rhs, and yields False otherwise.

SameQ requires exact correspondence between expressions, except that it considers Real numbers equal if their difference is less than the uncertainty of either of them.
2 === 2. gives False.
=== === gives True if all the are identical.
See The Mathematica Book on the web: Section 2.3.5.
See also: UnsameQ, Equal.