] yields True if no subexpression in expr matches form, and yields False otherwise.
] tests only those parts of expr on levels specified by levelspec.
form can be a pattern.
Example: FreeQ[f[x^2] + y^2, x^_].
FreeQ looks at the heads of raw expressions, testing whether those heads match form.
See the Mathematica book: Section 1.8.5, Section 2.1.5, Section 2.3.5.
See also: MemberQ, Count.
(See also the Further Examples for the Heads option.)
This example shows a feature of FreeQ that is absent from MemberQ: here, FreeQ recognizes that the sum a+c is a subexpression of the larger sum.
Here we see how the level argument to FreeQ affects its result. When no level is specified, matches at any level are reported.
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