 
Summary: Introduction to Coding Theory 89662
Final Exam, Moed Aleph 2009
Exam instructions: The exam is closed book: no material is allowed! Answer all questions and
formally prove all of your answers. The exam time is 2.5 hours.
Question 1 (25 points):
1. Formally define the notion of local decodability, and show that the WalshHadamard code is
2locally decodable with 1
4 .
2. Prove that if C is an [n, k] code such that C has distance d +1, then C is not ( 1)locally
decodable.
3. Prove that there exists a code of length n that is 1locally decodable for < 1
2 .
Question 2 (15 points): Show that if there exists a linear code C with parameters [n, k, d] where
d is even, then there exists a linear code C with parameters [n, k, d] such that every codeword has
even weight.
Question 3 (30 points): Let C be a binary linear code and denote by C the code derived by
taking the complement of all words in C.
1. Show that if the word (1, . . . , 1) C then C = C.
2. Prove or refute: C is a linear code.
3. Prove or refute: C C is a linear code.
