Question

[SOLVED] Number Theory Challenge (not actually that hard)

Find \(89^{307}\pmod{713}\) without using any kind of electronic aid.

Please at least attempt it before resorting to Wolfram.

Hint: \(713=23\cdot31\).

Answer 1

Since I ran out of space on the question: I only decided to ask this because when I was first doing this problem, I quickly discovered I didn't need an electronic aid.

Answer 2

Hint 2: Use the Chinese Remainder Theorem. You will find it isn't as bad as it usually is.

Answer 3

what is mod? what class do we learn about this?

Answer 4

It's a number theory function. So \(a\pmod{n}\) returns the remainder of some number a when divided by n. There are a bunch of interesting applications for this, and when \(n\) is prime, you get a bunch of awesome properties.

Answer 5

It's also called "Clock arithmetic" sometimes http://en.wikipedia.org/wiki/Modular_arithmetic

Answer 6

ahhh ok we used this in computer science

Answer 7

so 8mod(3) = 2?

Answer 8

Right.

Answer 9

hmm

Answer 10

one more hint, i dont see how the 23* 31 helps unless we could break up the top. into some multiple of 23 or 31

Answer 11

You need to look at \(89^{307}\pmod{23}\) and \(89^{307}\pmod{31}\) independently. Since these are prime, a bunch of nice properties apply, and you can simplify each to a number between 0 and 23/31. Once you have that, apply Chinese Remainder Theorem.

I've got to say though, if this is the first time you've ever really used modular arithmetic, this is still a hard problem.

Answer 12

thats the fun of it:) I got to eat but will be back in a bit to try more

Answer 13

Extra hints just for you. You need to be somewhat familiar with Fermat's Little Theorem. You also need to know how to find an inverse mod p using the Extended Euclidean Algorithm, and use successive squaring. These are all fairly basic number theory algorithms, and with a little practice, you should be able to get them.

Answer 14

\[89^{307}\pmod{23}=15\]\[89^{307}\pmod{31} = 15\]15?

Answer 15

Right on. Technically it requires an application of CRT at the end (which is not easy to do by hand), but since both are congruent to 15, you can skip that step.

Answer 16

Thank you for helping me through.

Answer 17

How much were you able to do by hand?

Answer 18

I used calculator for 13*12*20 (mod 23) and for verifying my answers.

Answer 19

Excellent job! However, there was an alternative, if slightly roundabout way of finding \(89^{307}\pmod{23}\) without using successive squaring.

Answer 20

oh hmm can you show me? or maybe a hint?

Answer 21

Basically, notice the following.\[89^{307}\equiv 20^{21}\pmod{23}\]By Fermat's Little Theorem, you have that \[20^{22}\equiv1\pmod{23}\]so\[20^{22}\equiv20^{21}\cdot20\equiv1\pmod{23}\]Hence, \[20^{21}\equiv20^{-1}\pmod{23}\]So you just need to solve the congruence\[20x\equiv1\pmod{23}\]

Answer 22

Got to go eat dinner now. Feel free to ask more questions on this and I'll answer them when I get back.

Answer 23

I like it, it's much better and simpler than mine. Thank you.

Answer 24

I love it when I can use it. It was a little trick I learned in my cryptography class.