k ∈N = 自然數???

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.

May be you could have some ideas from the following web site:
http://zh.wikipedia.org/w/index.php?title=Zermelo_%E9%9B%86%E5%90%88%E8%AE%BA&variant=zh-hk

WOW~!@@

k∈R:k係實數