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I could not figure out the solution of $(x^2- 1) \bmod 8= 0$ Thank you.

Julian Kuelshammer
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Jennifer
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    You cannot solve a term. There is no equation to solve.. – user127.0.0.1 Dec 01 '12 at 09:44
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    Do you perhaps mean $x^2\equiv 1\pmod 8$? – Brian M. Scott Dec 01 '12 at 09:48
  • sorry, it should be x^2-1 mod 8 = 0 – Jennifer Dec 01 '12 at 12:44
  • Welcome to math.SE: since you are new, I wanted to let you know a few things about the site. In order to get the best possible answers, it is helpful if you say in what context you encountered the problem, and what your thoughts on it are; this will prevent people from telling you things you already know, and help them give their answers at the right level. – Julian Kuelshammer Dec 01 '12 at 14:39

3 Answers3

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There are only $8$ possibilities. Counting with $a^2=(-a)^2$ for all numbers $a$, it can be reduced to $5$. $$0^2=0\equiv 0,\quad (\pm 1)^2=1\equiv 1, \quad (\pm 2)^2=?,\quad (\pm 3)^2=?,\quad 4^2=16\equiv 0 \pmod 8.$$

Berci
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If $2^{3+n}\mid (x^2-1)$ where $n\ge 0$

Clearly, $x$ is odd and $2^{n+1}\mid\left(\frac{x+1}2\right)\left(\frac{x-1}2\right) $

We know using Bézout's Identity, $\left(\frac{x+1}2,\frac{x-1}2\right)\mid\left(A \frac{x+1}2+B\frac{x-1}2\right)$ where $A,B$ are integers.

But $\frac{x+1}2-\frac{x-1}2=1,$ putting $A=1,B=-1$
so $(\frac{x+1}2,\frac{x-1}2)\mid 1\implies (\frac{x+1}2,\frac{x-1}2)=1$

So, either $2^{n+1}\mid\frac{x-1}2$ or $2^{n+1}\mid\frac{x+1}2$

(a)If $2^{n+1}\mid\frac{x-1}2, 2^{n+2}\mid(x-1),x\equiv1\pmod {2^{n+2}},x=1+a2^{n+2}$ where $a$ is any integer.

If $1+a_12^{n+2}\equiv1+a_22^{n+2}\pmod {2^{n+3}}\iff a_1\equiv a_2\pmod 2$

If $a$ is even $=2b$(say,) $x=1+(2b)2^{n+2}\equiv1\pmod {2^{n+3}}$

If $a$ is odd $=2c+1$(say,) $x=1+(2c+1)2^{n+2}\equiv1+2^{n+2}\pmod {2^{n+3}}$

(b)Similarly, if $2^{n+1}\mid\frac{x+1}2$ we shall get two solutions namely, $-1,2^{n+2}-1\pmod {2^{n+3}}$

So, there will be $4$ in-cogruent solutions $\pmod {2^{n+3}}$

lab bhattacharjee
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  • Honestly, I don't think that such a complicated solution can help the OP, if they're not able to instantly see the solutions of the question. – yo' Dec 02 '12 at 18:55
  • @tohecz, this is a generalization. We can safely put $n=0$ from the start to avoid complications. Though small numbers like $8$ can be handled directly using trial with $(x,8)=1$ – lab bhattacharjee Dec 02 '12 at 18:58
  • Then you do it in an unbelievably complicated way IMHO. – yo' Dec 02 '12 at 19:21
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Let $(x^2-1)\equiv0\pmod{2^k}$ with $k\geq3$, i.e. $x^2-1=2^kn$ for some $n$.

We have $2^kn=x^2-1=(x+1)(x-1)$. Since the left-hand side is even and the numbers on the right-hand side have the same oddity, they are both even. At the same time, they differ by $2$ so the number $4$ divides only one of them.

Therefore they are of the form $\{x+1,x-1\}=\{n_1,2^kn_2\}$ or $\{2n_1,2^{k-1}n_2\}$, where $n=n_1n_2$ is some factorization of $n$. From this, we have $x=2^{k-1}m\pm1$ where $m$ is some integer. Four of them are in the interval $[0,2^k)$:

$$1, 2^{k-1}-1, 2^{k-1}+1, 2^k-1.$$

Easy calculation shows that all these really solve the equation. For the case $k=3$ this gives $x\equiv1,3,5,7 \pmod8$.

yo'
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