# ECE 404 Homework #3 solution

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## Description

This homework assignment will be focusing on topics related to group theory and finite fields.
It involves programming as well as a part on theoretical problem-solving.
Theory Problems
Solve the following problems.
1. Show whether or not the set of remainders Z21 forms a group with the modulo addition
operator. Then show whether or not Z21 forms a group with the modulo multiplication
operator.
2. Is the set of all unsigned integers W a group under the gcd(·) operation? Why or why not?
(Hint: Find the identity element for {W, gcd(·)}.)
3. Compute gcd(21609, 18432) using Euclid’s algorithm. Show all of the steps.
4. Use the Extended Euclid’s Algorithm to compute by hand the multiplicative inverse of 24 in
Z35. List all of the steps.
5. In the following, find the smallest possible integer x. Briefly explain (i.e. you don’t need to
list out all of the steps) how you found the answer to each. You should solve them without
simply plugging in arbitrary values for x until you get the correct value:
(a) 6x ≡ 3 (mod 23)
(b) 7x ≡ 11 (mod 13)
(c) 5x ≡ 7 (mod 11)
Programming Problem
Rewrite and extend the Python (or Perl) implementation of the binary GCD algorithm presented
in Section 5.4.4 so that it incorporates the Bezout’s Identity to yield multiplicative inverses. In
other words, create a binary version of the multiplicative-inverse script of Section 5.7 that finds
the answers by implementing the multiplications and division as bit shift operations.
Your script should be named mult inv.py/pl and accept two command-arguments:
mult_inv.py a b
Which should print the multiplicative inverse of a mod b
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Submission Notes
• For this homework you will be submitting 2 files electronically. Your submission must include:
– A PDF containing your answers to the theory problems. You are allowed to include
scans or photos of handwritten work in the PDF, but your work must be clearly legible.
– The file mult inv.py/pl containing your code for the programming problem.
Electronic Turn-in
turnin -c ece404 -p hw03 hw03.pdf mult_inv.pl (if using Perl)
turnin -c ece404 -p hw03 hw03.pdf mult_inv.py (if using Python)
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