Question

What does the expression `x in RR` mean ?

Answer 1

A real number is any rational or irrational number.

For example: `pi, e,2, 4, -78, 1/2, 23/6` and so on

Answer 2

It means that `x` is an element of the set of real numbers which we symbolize with `R`.

Answer 3

It usually means:

"`x` is a member of the set of real numbers"

or more simply:

"`x` is a real number"

Explanation:

• `RR` usually denotes the set of Real numbers.

• `in` denotes membership.

So `x in RR`, means that `x` is a member of the set of Real numbers. In other words, `x` is a Real number.

Related expressions are:

• `AA x in RR" "` meaning "for all `x` in the set of real numbers". in other words: "for all real numbers `x`".

• `EE x in RR : ..." "` meaning "there exists a member `x` in the set of real numbers such that ..." or "there exists a real number `x` such that ...".

In some kinds of constructive mathematics, where speaking of "the set of Real numbers" is considered a little presumptuous, the expression "`x in RR`" may be read as "`x` is a real number" and `AA x in RR` understood as "for any real number `x`", etc., avoiding the concept of a completed set of real numbers.