1. ## Divisibility by 121

Can someone help me to prove that $n^2+3n+5$ is not divisible by 121 for any $n \in Z$ ?

2. Originally Posted by OReilly
Can someone help me to prove that $n^2+3n+5$ is not divisible by 121 for any $n \in Z$ ?
Suppose that for some $n \in \mathbb{Z}$ that $n^2+3n+5$ is divisible by $121$
then the exists a $\kappa$ such that:

$
n^2+3n+5=121\ \kappa
$

So from the quadratic formula we have:

$
n=\frac{-3 \pm \sqrt{9-4(5-121\ \kappa})}{2}
$

Which implies that $\sqrt{9-4(5-121\ \kappa})$ is an odd integer, $m$ say. So:

$
121\times4\times \kappa - 11=m^2
$

but the LHS is divisible by $11$, and as $m^2$ is a square it is also divisible
by $121$ (which is $11^2$), but that would imply that $11$ is divisible by $121$; a
contradiction, so no such $n$ exits.

RonL

3. Can you just explain me this part, I didn't quite understand it.

Originally Posted by CaptainBlack

$
121\times4\times \kappa - 11=m^2
$

but the LHS is divisible by $11$, and as $m^2$ is a square it is also divisible
by $121$ (which is $11^2$), but that would imply that $11$ is divisible by $121$; a
contradiction, so no such $n$ exits.

RonL

4. Can someone help me to prove that $n^2+3n+5$ is not divisible by 121 for any $n \in Z$ ?
If $X = n^2+3n+5$ is divisible by 121 then so is $4X = 4n^2+12n+20 = (2n+3)^2 + 11$. If $4X$ is divisible by 121 then it's divisible by 11 and hence 11 divides $(2n+3)^2$. If 11 divides a perfect square $(2n+3)^2$ then it divides $2n+3$: and that implies that 121 divides $(2n+3)^2$. So we have 121 dividing $(2n+3)^2$ and $(2n+3)^2+11$, hence the difference, which is 11. But that's a contradiction.

5. Can you just explain me this part, I didn't quite understand it.

Originally Posted by CaptainBlack

$
121\times4\times \kappa - 11=m^2
$

but the LHS is divisible by $11$, and as $m^2$ is a square it is also divisible
by $121$ (which is $11^2$), but that would imply that $11$ is divisible by $121$; a
contradiction, so no such $n$ exits.

RonL
If a square is divisible by a prime, it is also divisible by the square of the
prime. It's a consequence of the fundamental theorem of arithmetic, that
the decomposition of a number into a product of primes is essential unique.

So the left hand side of the equation being divisible by 11 implies that the
right hand side is also divisible by 11, but the right hand side is a square.
So the right hand side is divisible by 121 (the square of 11). This implies
that the left hand side is divisible by 121, and so that 11 is divisible by

RonL

6. When I was trying to solve the problem I have done this:
$\begin{array}{l}
n^2 + 3n + 5 = 121k \\
n^2 - 8n + 11n + 16 - 11 = 121k \\
(n - 4)^2 + 11n - 11 = 121k \\
(n - 4)^2 = 121k - 11n + 11 \\
(n - 4)^2 = 11(11k - n + 1) \\
\end{array}
$

But I have stoped there.

Let me see if I have understood good.
$11(11k - n + 1)$ is obviosly divisible by 11, so $(n - 4)^2$ is also divisible by 11. But $(n - 4)^2$ is also divisible by $11^2$ which now implies that $11(11k - n + 1)$ is divisible by 121. So we get that $\frac{{11}}{{121}} = \frac{{11k - n + 1}}{{(n - 4)^2 }}$ which shows that 11 is not divisible by 121.

Is that correct?

I didn't know that if a square is divisible by a prime, it is also divisible by the square of the prime. Boy, I feel stupid and embarrassed now!

Divisibility of numbers is simply killing me, I just can't solve any harder problem. I am not expert, but I think that in order to solve those problems you need to know Number theory. Since I am on level of Algebra 1 and 2 for high school (problem that I have posted is from Algebra 1) I find that many harder problems constructed for learning Algebra 1 and 2 that concerns divisibility are based on writers thorought knowledge of Number theory. I don't know if I am wrong, but that's my impression. Either that, or I must find excuse!

7. Originally Posted by OReilly
I didn't know that if a square is divisible by a prime, it is also divisible by the square of the prime.
Proof,
Let $p|ab$ then, $p|a$ or $p\not | a$. If $p\not |a$ then, $\gcd(a,p)=1$ then, $p|b$ by Euclid's Lemma. Thus, $p|a \mbox{ or }p|b$.
Thus, given $p|a^2$ you have, $p|aa$ thus, $p|a$ or $p|a$ thus, $p|a$ definetly. Now, if $c|d$ then, $c^n|d^n$ but since $p|a$ then $p^2|a^2$ if you want to be formal about it

Originally Posted by OReilly
Boy, I feel stupid and embarrassed now!
That is right keep on feeling stupid.

8. When I was trying to solve the problem I have done this:
$\begin{array}{l}
n^2 + 3n + 5 = 121k \\
n^2 - 8n + 11n + 16 - 11 = 121k \\
(n - 4)^2 + 11n - 11 = 121k \\
(n - 4)^2 = 121k - 11n + 11 \\
(n - 4)^2 = 11(11k - n + 1) \\
\end{array}
$

But I have stoped there.

Let me see if I have understood good.
$11(11k - n + 1)$ is obviosly divisible by 11, so $(n - 4)^2$ is also divisible by 11. But $(n - 4)^2$ is also divisible by $11^2$ which now implies that $11(11k - n + 1)$ is divisible by 121. So we get that $\frac{{11}}{{121}} = \frac{{11k - n + 1}}{{(n - 4)^2 }}$
I don't see how this last equation follows from what you say it does.

which shows that 11 is not divisible by 121.

Is that correct?

I didn't know that if a square is divisible by a prime, it is also divisible by the square of the prime. Boy, I feel stupid and embarrassed now!
RonL

9. Originally Posted by CaptainBlack
I don't see how this last equation follows from what you say it does.
RonL
My lack of understanding again!

Let me try again.

From my example we have:
$\begin{array}{l}
(n - 4)^2 - 11 = 121k - 11n \\
(n - 4)^2 - 11 = 11(11k - n) \\
\end{array}
$

That implies that $(n - 4)^2 - 11$ is divisible by 11. But $(n - 4)^2$ is also divisible by 121 but we would have that $(n - 4)^2 - 11$ is also divisble by 121, which is different.

Is that ok?

10. Not necessarily, you proof falls:
But $(n-4)^2$ is also divisible by 121 but we would have that $(n-4)^2-11$.is also divisble by 121
Note if $(n-4)^2=121$ then, $(n-4)^2-11=110$ which is not divisible by 121. Thus, that statement is false.

11. Originally Posted by ThePerfectHacker
Not necessarily, you proof falls:

Note if $(n-4)^2=121$ then, $(n-4)^2-11=110$ which is not divisible by 121. Thus, that statement is false.
That was what I have said. I said "which is different" after "is also divisble by 121". I should have said "which is contradictory".

12. If,
$a|b,a\not |c$ then, $a\not |(b+c)$

According to you,
$a|b,a\not |c$ then, $a|(b+c)$

Where,
$a=121$
$b=(n-4)^2$
$c=11$

Again in Line,
But $(n-4)^2$ is also divisible by 121 but we would have that $(n-4)^2-11$.is also divisble by 121

13. Originally Posted by ThePerfectHacker
If,
$a|b,a\not |c$ then, $a\not |(b+c)$

According to you,
$a|b,a\not |c$ then, $a|(b+c)$

Where,
$a=121$
$b=(n-4)^2$
$c=11$

Again in Line,
No, I said exactly what you have said. That is NOT divisible not that is divisible. Maybe I didn't wrote it good.

14. From the equation,
$
\begin{array}{l} (n - 4)^2 - 11 = 121k - 11n \\ (n - 4)^2 - 11 = 11(11k - n) \\ \end{array}
$

1)RHS is divisible by 11,
2)So LHS is divisible by 11,
3) $11|((n-4)^2-11)$
4)Since, $11|(-11)$ so, $11|(n-4)^2$
5)So, 121 divides $(n-4)^2$
6)But then, 121 divides 11.

Explain to me step 6!

15. Originally Posted by ThePerfectHacker
From the equation,
$
\begin{array}{l} (n - 4)^2 - 11 = 121k - 11n \\ (n - 4)^2 - 11 = 11(11k - n) \\ \end{array}
$

1)RHS is divisible by 11,
2)So LHS is divisible by 11,
3) $11|((n-4)^2-11)$
4)Since, $11|(-11)$ so, $11|(n-4)^2$
5)So, 121 divides $(n-4)^2$
6)But then, 121 divides 11.