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- 1 decade agoFavorite Answer
My dear cousin

Seems that you are not familar to Set theory

Let me introduce you this topic

A set can be interpreted as a bag containing some objects

These objects are called "the elements" of the set.

e.g. Let S be the set of atoms in the whole world, then every single atom is an element of S.

Back to your question,

mathematicians use spectial symbols (N, I, Q, R and C) to show the set of Natural no.s, intergers, Rational no.s, Real no.s and complex no.s.

The expression " a ∈S " means " a is an element of the set S".

Therefore,

k ∈N means k is a natural no.

k ∈I means k is an integer

k ∈Q means k is a rational no.

k ∈R means k is a real no.

k ∈C means k is a complex no.

ect.

If you are interested in Set theory, you may read the book "Stories about Sets (集的故事)", it is a very good book to beginners.

At last, I would like to conclude with 2 question

i )Do the set N has less elements than that in I?

ii)Do the set I has less elements than that in Q?

The answer of this two questions can be found in the book mentioned above.