Read Example 4 on p.532 of the textbook. Show work, and be sure to answer all parts of this question. (a) In this example
a n
​
represents the number of valid
n
-digit codewords. How is a valid codeword defined for this example? (b) What is the recurrence relation for
a n
​
? (c) Given
a 1 =9
, find
a 2
​
and
a 3
​