Need descrete math help

  • #1

Homework Statement


Show that n2 [tex]\neq[/tex]2 (mod6) for all n in Z



Homework Equations





The Attempt at a Solution



0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 1 2 3 4 5
2 0 2 4 0 2 4
3 0 3 0 3 0 3
4 0 4 2 0 4 2
5 0 5 4 3 2 1

I did only the table for mod 6 and then I don't have an idea what to do.
I am not even sure if I understand what exactly I have to do with this problem.
Please help me if you can.
 
Last edited:

Answers and Replies

  • #2
gabbagabbahey
Homework Helper
Gold Member
5,002
7
Hmmm....have you tried proof by contradiction? That is, assume that [itex]n^2 \equiv 2 \pmod{6}[/itex]....what does that imply?
 
  • #3
614
0
I did not attempt a solution but you might want to rewrite the statement.

n2 congruent to 2 mod 6 is the same as n2 - 2 is a multiple of 6. So you might want to define f(n) = n2 - 2 and show what happens when you divide f(n) by 6.
 
  • #4
I think the only hint I get for this was the reminder needs to be [tex]\neq[/tex]2.

I am gessing that has something to do with division Algorithm. I will try the above ideas.
 
  • #5
35,638
7,510
VeeEight said that "n[tex]^2[/tex] congruent to 2 mod 6 is the same as n[tex]^2[/tex] - 2 is a a multiple of 6."

That's also the same as saying that n[tex]^2 - 2 \equiv[/tex] 0 mod 6.

This one is ripe for a proof by induction.
 
  • #6
I don't think I know how to do it by induction. Thank you. I will try and I will come back again.
 
  • #7
35,638
7,510
Pick a value of n for which your statement is true, such as n = 2.

Assume that for n = k, your statement is true. IOW, assume that k^2 != 2 mod 6.
Now show that for n = k + 1, (k + 1)^2 != 2 mod 6, using the induction hypothesis (the thing you assumed in the previous step).
 

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