Does a mod p mod q = a mod q mod p?

Question

How to prove/disprove that (a mod p) mod q = (a mod q)mod p ?

Supposing $p < q$, we have 3 situations:

$a{\space}{\epsilon}{\space}[0, p)$, obviously the statement is true for this case
$a{\space}{\epsilon}{\space}[p, q)$

$a{\space}mod{\space}p{\space}mod{\space}q=(a-a/p)mod{\space}q=a{\space}mod{\space}q-a/p{\space}mod{\space}mod{\space}q=a-a/p$

and

$a{\space}mod{\space}q{\space}mod{\space}p=a{\space}mod{\space}p=a-a/p$

and $a > q$ which reduces to first two situations.

Taking these three situation I have found that the statement is true. I am missing something ?

score 3 · Answer 1 · answered Mar 31 '16 at 07:31

3

Your solution is pretty sloppy, and as usual, sloppiness leads to wrong results.

Yes, if $a<p$, the equality holds, but it isn't enough to just say "obviously."
Your solution for $p\leq a < q$ makes no sense. I have no idea what you were trying to prove.
How exactly does $a>q$ reduce to the first two situations? You can't just say it does without proving it.

Also:

Try the values of $a=10, p=10, q=3$.

answered Mar 31 '16 at 07:31

5xum

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A number theorist somewhere dies when people write things like let $p=10$. :) – Arkady Mar 31 '16 at 07:35
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@Jake Well OP didn't specify if $p$ is prime. But even so, $a=p=3$ works just as well if $q=2$. – 5xum Mar 31 '16 at 07:37
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I know, I know. I was just making a joke. :) +1 – Arkady Mar 31 '16 at 07:37
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@Jake I thought so. In fact, I was sure with a $p$ value of $<0.05$ :). – 5xum Mar 31 '16 at 07:38
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Oh wow. That just took it to a whole new level. – Arkady Mar 31 '16 at 07:44

score 2 · Answer 2 · answered Mar 31 '16 at 08:45

2

There is no need to go very far:

$$3\bmod 2\bmod 3=1,\\3\bmod3\bmod2=0.$$

answered Mar 31 '16 at 08:45

score 1 · Answer 3 · answered Mar 31 '16 at 07:41

I do not think talking about an element modulo $p$ and then modulo $q$ makes much sense because when we talk about $3\pmod 7$ we are simultaneously talking about all integers that leave a remainder of $3$ when divided by $7$. This includes numbers like $3,10,17$ etc.

So if you want to talk about $3 \pmod 7 \pmod 2$, should this be $1$ because $3$ is $1\pmod 2$ or should this be $0$ because $10$ is $0\pmod 2$? It's pure chaos.

Does a mod p mod q = a mod q mod p?

3 Answers3