Recursion relation and initial conditions Write a recursion relation and initial conditions for the number of words of length n using the letters A,B,C  such that the number of the letter 'A' is even. 
(A occur an even number of times)

Hint:
You can add an 'A' to a string with an odd number of 'A's, or you can add a 'B' or a 'C' to a string with an even number of 'A's.
The number of strings with an odd number of 'A's can be found directly from the number of strings with an even number of 'A's.
