Reading
Section 2.3 of Theory of Computing, A Gentle Introduction
Homework
Printout the homework and answer the questions on that paper.

Review of NDFA-to-DFA conversion so far
Last class we introduced this amazing algorithm that converts an NDFA without $lambda$-transitions to a DFA that accepts the same language. For review, here it is:
Algorithm: Non-Deterministic (w/o λ-transitions) to Deterministic FA
Input:M = (Q,Σ,Δ,s,W), where Δ contains no λ-transitions
Output:M' = (2Q,Σ,δ, {s}, {R ∈2Q | R ∩ W ≠ ∅ }), where
 δ : 2Q x Σ → 2Q δ(H,x) = { q ∈ Q | (p,x,q) ∈ Δ for some p ∈ H }

For further practice, run the above algorithm on the machine M = ({q0,q1},{a,b,c},{(q0,a,q0},(q0,c,q0),(q0,c,q1),(q1,a,q1),(q1,b,q1)},q0, {q1}).

What about the λ-transitions?
Our algorithm for converting non-deterministic finite automata into deterministic finite automata doesn't work if we have λ-transitions. So, to round things out for us, we need an algorithm that takes a nondeterministic machine with λ-transitions and produces a new nondeterministic machine that accepts the same language, but which has no λ-transitions. The general idea is this: Suppose state p has an λ-transition to state q, and suppose that for character x you have transitions from state q to states r1, r2, ..., rk. Then essentially, if you're in state p and x is the next character in the input, you can either take any of p's x-transitions or you can go to q and then take one of q's x-transitions, which take you to r1, or r2 or ... or rk. Thus, we can replace the λ-transition (p,λ,q) with the regular transitions {(p,x,r) | x ∈ Σ and (q,x,r) ∈ Δ}.}

Now, this idea works fine if there are not, in turn, λ-transitions out of q. The question is, if a machine has λ-transitions, can we always find at least one λ-transition to a state that has no λ-transitions? Then we could keep eliminating λ-transitions of that type until there weren't any at all. Unfortunately, there is one way that we could be unable to find an λ-transition to a state out of which there are no λ-transitions: What if there is a loop (or in graph parlance a cycle) of λ-transitions? So, first we need to get rid of any cycles of λ-transitions. Well, if there is a cycle of λ-transitions, the states in the cycle all form one big "super state", because whenever you're in one of the states you can get to any other without reading any input. So if there's a cycle, we'll merge all the states along the cycle into one state, wiping out all the λ-transitions between them in the process. This can simply be repeated until we finally arrive at a machine that has no cycles.

Algorithm: Lose λ-transition
Input: NDFA M = (Q,Σ,Δ,s,W) and (p,λ,q) ∈ Δ, where there are no λ-transitions out of q.
Output: NDFA M' such that $L(M) = L(M')$ and $M'$ has one fewer $\lambda$-transitions than $M$
M' = (Q,Σ,Δ',s,W'), where
Δ' = Δ - {(p,λ,q)} ∪ { (p,x,r) ∈ QxΣxQ | (q,x,r) ∈ Δ }
W' = W ∪ {p} if q ∈ W, and W otherwise

Claim: Algorithm Lose λ-transition meets its specification, i.e. L(M') = L(M) and there is one fewer λ-transition in M' than in M.
Proof: The fact that there is one fewer λ-transition in M' than in M is obvious, so we concentrate on proving that L(M') = L(M). We need to prove that for any string $w$, $w \in L(M)$ if and only if $w \in L(M')$. This means that $M$ and $M'$ accept the same strings and reject the same strings.

• If $w \in L(M)$ then $w \in L(M')$: Since $w \in L(M)$, we know that $(s,w) \Rightarrow_M^* (r,\lambda)$, where $r \in W$. In other words, we have a (potentially) long sequence $(s,w) \Rightarrow_M \ldots \Rightarrow_M (r,\lambda)$ representing a valid, accepting computation in $M$. To prove that $w$ is accepted by $M'$, I will give you an algorithm that converts that sequence into a sequence representing an accepting computation for string $w$ and machine $M'$. Here it is:
1. if the sequence ends in $(p,\lambda) \Rightarrow_M (q,\lambda)$, drop the "$\Rightarrow_M (q,\lambda)$".
Note: this removes any use of $(p,\lambda,q)$ as the last step in the computation.
2. moving left-to-right in the sequnce, replace any $(p, xu) \Rightarrow_M (q,xu) \Rightarrow_M (r,u)$ with $(p, xu) \Rightarrow_M (r,u)$
Note: this removes any remaining use of $(p,\lambda,q)$
3. change any remaining $\Rightarrow_M$ to $\Rightarrow_{M'}$
The resulting sequence is a valid computation in $M'$, and it either ends in the same state $r$, which is accepting for $M'$, or the original ended in $q$ and the new sequence ends in $p$. But in this case $q$ must have been accepting for $M$, which means $p$ is accepting in $M'$. In either case, $w$ is accepted by $M'$, which is what we needed to show.
• If $w \in L(M')$ then $w \in L(M)$: I'll leave this up to you. You need an algorithm that converts a sequence $(s,w) \Rightarrow_{M'} \ldots \Rightarrow_{M'} (r,\lambda)$ where $r \in W'$ to a sequence representing an accepting computation in $M$.

The only problem that remains is what to do if there are $\lambda$-transitions, but all of them into states that themselves have outgoing $\lambda$-transions. In this case, we can't apply the above algorithm. The thing to note is that this can only happen in the presence of cycles of $\lambda$-transitions. The next algorithm removes such cycles by merging all states in such a cycle into a single state.

Algorithm: Lose λ-cycle Input: NDFA M = (Q,Σ,Δ,s,W) and states p1,p2,...,pk comprising a cycle of 2 or more λ-transitions in M
Output: NDFA M' such that $L(M') = L(M)$ and $M'$ has fewer $\lambda$-transitions than $M$
M' = (Q',Σ,Δ',s',W') where
Q' = Q - {p1,...,pk} ∪ {D}, where D is a new state i.e. D ∉ Q
 Δ' = Δ - {p1,...,pk}x(Σ ∪ {λ})xQ remove transitions out of the cycle - Qx(Σ ∪ {λ})x{p1,...,pk} remove transitions into the cycle ∪ {(D,x,q) ∈ {D}x(Σ ∪ {λ})xQ' | for some pi, (pi,x,q) ∈ Δ} add transitions out of D ∪ {(q,x,D) ∈ Q'x(Σ ∪ {λ})x{D} | for some pi, (q,x,pi) ∈ Δ} add transitions into D ∪ {(D,x,D) ∈ {D}xΣx{D} | for some pi and pj, (pi,x,pj) ∈ Δ} add D's loops
s' = D if s ∈ {p1,...,pk}, and s otherwise
W' = W - {p1,...,pk} ∪ D if W ∩ {p1,...,pk} = ∅, and W otherwise
Claim: Algorithm: Lose λ-cycle meets its specification, i.e. L(M') = L(M) and there are fewer λ-transitions in M' than in M.
Proof: In this case we will rely on the in-class discussion to convince you that this algorithm works.

A full algorithm converting an NDFA into a DFA
Assume you're given a machine M = (Q,Σ,Δ,s,W) which has no transitions of the form (q,λ,q). This assumption is no real restriction, since such transitions are pointless and can simply be removed without affecting the language accepted by the machine. Set M' = M and then:

1. Apply Algorithm: Lose λ-cycle repeatedly until there are no λ-cycles left in M'. This will only require a finite number of applications of the algorithm, since the number of λ-transitions decreases with every application of the algorithm.
2. Repeatedly apply Algorithm Lose λ-transition until there are no λ-transitions left. This can only take a finite number of steps because the number of λ-transitions is finite, and each application of the algorithm reduces the number of λ-transitions. We are assured that we can always find a transition (p,λ,q) such that no λ-transitions come out of q because the machine has no λ-cycles. (Think about it!) Moreover, since we don't add new λ-transitions with this algorithm, no new cycles can appear.
3. Apply the algorithm from Class 14 to convert the resulting nondeterministic machine (which now has no λ-transitions) into an equivalent deterministic finite automaton.

The deterministic finite automaton that results from this process accepts the same language as the original nondeterministic machine!

Christopher W Brown
Last modified: Mon Sep 28 15:55:53 EDT 2009