Solve congruence for unknown power

Question

QN: Solve $8^x \equiv 3 $ mod 43.

I am inspired by the method here: (https://math.stackexchange.com/a/1332788/737799) However there seems to be no solutions in this case.

Firstly convert both sides of the congruence to base-8. I have found that $3^{39} \equiv 8$ mod 43. Then $8^{39x} \equiv 3^{39} \equiv 8$ mod 43.

Then solving $39x \equiv 1$ mod $\phi(43)=42$. There is no solution to this congruence.

However the theorem (also in the link) for the last step states: if is a primitive root modulo , then $a^x\equiv a^y $ mod p if and only if $x\equiv y$ mod $\phi(p)$. For this problem 8 is not a primitive root mod 43. What can I do in this case? Thanks in advance!

See here for one simple way to compute solutions when they exist (Shanls's baby-giant step). — Bill Dubuque, Sep 27 '20 at 02:17

markvs · Answer 1 · 2020-09-27T01:57:50.097

1

We have

$$8^{x}\equiv 8, 21, -4, 11, 2, 16, -1,-8, -21, 4, -11, -2, -16, 1, 8...\mod 43$$ as $x=1, 2,...$

So $8^x$ is never $3\mod 43$.

edited Sep 27 '20 at 01:57

answered Sep 27 '20 at 01:51

markvs

19,653

Bill Dubuque · Accepted Answer · 2020-09-27T02:09:52.853

1

$\bmod 43\!:\ \left[2^{\large 3x}\equiv 3\right]^{\large 14}\,\overset{\rm Fermat}\Longrightarrow\ 1\equiv 3^{\large 14}\equiv 36\,\Rightarrow\!\Leftarrow$

edited Sep 27 '20 at 02:09

answered Sep 27 '20 at 01:58

Bill Dubuque

272,048

i.e. raise both sides of the congruence to power $14,, $ using $,2^{\large 42}\equiv 0,$ by Fermat. – Bill Dubuque Sep 27 '20 at 02:08

Solve congruence for unknown power

2 Answers2