12-240/Fields' Further proof

In the first class, Professor says something about particular fields. Forgive me, because I am an international student, if I can not express information precisely.

About: F(n) F(1) F(2) and F(3) are a field, but F(4) is not. Professor said that any number N which is not a prime number can not "form" a field. We should ask why...

Here is the proof.

If we have a field F(n), and n=a*b (a,b,n∈ N*, a,b≠1)

IN defination of multiplication

• 0 1 2 3 .......... b.......n-1

0 . . . . ......................

1 . . . . ......................

2 . . . . ......................

3 . . . . .......................

4 . . . . .......................

.

a 0 (a) (2a) (3a)...........(a*b).....(n-1)*a ( in this row, every element mod n)

.

.

.

.

n-1..................................

see the (a+1)th row

If F(n) is a field, then

 1.m*a=k*n+1   (k,m∈N*, m<n)
 2.n=ab

==>>m=kb+1/a

unless a=1

     m will not exist.
   so F(n), when n is not a prime number, is not a field.

                 There is a large need for me to improve my format. Editing is welcomed.

  PS: But till now, there are still some questions existing.
      How can we prove that a prime number N can absolutely form a field? Is there any exception?
      I am still working on it.
                                                                ----Michael.Wang