Science:Math Exam Resources/Courses/MATH312/December 2009/Question 03 (b)

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MATH312 December 2009

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Question 03 (b)
Find $\displaystyle 9^{23}\mod {31}$

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

Hint
Here the square multiply algorithm might be the best approach.

Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

We use the square multiply algorithm to solve this problem. We compute $\displaystyle 9^{2a}$ for a up to 4 to compute this value. We see that $\displaystyle {\begin{aligned}9&\equiv 9\mod {31}\\9^{2}&\equiv 81\equiv 19\mod {31}\\9^{4}&\equiv (9^{2})^{2}\equiv 19^{2}\equiv 361\equiv 51\equiv 20\mod {31}\\9^{8}&\equiv (9^{4})^{2}\equiv 20^{2}\equiv 400\equiv 90\equiv 28\equiv -3\mod {31}\\9^{16}&\equiv (9^{8})^{2}\equiv (-3)^{2}\equiv 9\mod {31}\end{aligned}}$ Now, we have that $\displaystyle {\begin{aligned}9^{23}&\equiv 9^{16}\cdot 9^{4}\cdot 9^{2}\cdot 9^{1}\mod {31}\\&\equiv 9\cdot 20\cdot 19\cdot 9\mod {31}\\&\equiv 9\cdot 9\cdot 19\cdot 20\mod {31}\\&\equiv 9^{2}\cdot 19\cdot 20\mod {31}\\&\equiv 19\cdot 19\cdot 20\mod {31}\\&\equiv 19^{2}\cdot 20\mod {31}\\&\equiv 20\cdot 20\mod {31}\\&\equiv 28\mod {31}\end{aligned}}$

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