Digraphs:-
Encoding single letters one by one is rather limiting, since there are only 26 possible messages. It is better to use longer basic messages. One way is to send letters in pairs, such as ab or kn. These pairs are termed digraphs, and it is clear that there a total of 262 = 676 of them. To be practical, there must be a simple formula for assigning digraphs to numbers. We assign aa → 0, ab → 1, ... , az → 25, ba → 26, etc.
(a) What is the number corresponding to the digraph th?
(b) What is the general procedure for finding the number corresponding to a digraph?