In: Computer Science
Problem 14.1.2 Let M be a DFA with transition function δ. Prove carefully, by induction for all strings w over M’s alphabet, that δ ∗ (i, w) = j if and only if there is a path from node i to node j in the graph of M such that the labels on the edges of the path, read in order, form w.
Base case :- |w| = 1.
Let w= a.
Then
Clearly for |w|= 1,
only if there is path (which is single edge in this case) in graph
of M from i to j with label of edge being single letter w
itself.
Hence the statement is true for base case.
Induction hypothesis :-
if and only if there is path from i to j in the graph of M with
labels on the edges of path in order form string w and |w| <=
k.
Inductive step :- |w| = k+1
Let w = w'c where
Now if
Since |w'| = k. If
then as per induction hypothesis , there must be path from node i
to node k in graph of M with labels of edges being alphabets in w'
from left to right.
Now, 
This means that there is path from node i to node k and then there is an edge from node k to node j with label of edge being alphabet c. This means that there is path from node i to node j of length k+1.
Hence the argument that
if and only if there is path from node i to node j in the graph of
M satisfied even for |w|=k+1 which proves that given statement is
correct for all string w.
Please comment for any clarification .