like
:"x is an aunt of y if, for some person z, z is a parent of y, and x is a sister of z"(with free variables x, y, and bound variable z) defining the notion of "aunt" in terms of "parent" and "sister". Another, more formal example, which defines the property of being a prime number, is
:"P(x) if ?m,n?: m>1 ? n>1 -> x!= m·n",(with free variable x and bound variables m,n).

Fermat example

An example of a closed formula with truth value false involves the sequence of Fermat numbers

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