# k ∈N = 自然數???

k∈N 是解k是自然數???

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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 &quot;the elements&quot; 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.

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 &quot; a ∈S &quot; means &quot; a is an element of the set S&quot;.

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 &quot;Stories about Sets (集的故事)&quot;, 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_...

• k∈R:k係實數

Source(s): me