The equivalence studied up to now is quite discriminating, in the sense that it distinguishes, for example, τ.P and τ.τ.P.

- If an external observer can count the number of non-observable actions (i.e., the τ’s), this distinction makes sense.
- If we assume that an observer cannot access any internal information of the system, then this distinction is not acceptable.

The idea of the new equivalence is to ignore (some) τ’s:
- a visible action must be replied to with the same action, possibly together with some internal actions
- an internal action must be replied to by a (possibly empty) sequence of internal actions.

We define the relation $\implies$ as:

$P \implies P'$ if and only if there exist $P_{0}, P_{1},\dots,P_{k}$ (for $k \geq 0$) such that $P=P_{0} \xrightarrow{\tau} P_{1} \xrightarrow{\tau}\dots\xrightarrow{\tau}Pk=P'$

relation $\xRightarrow{\hat{\alpha}}$:
- if $\alpha=\tau$ then $\xRightarrow{\hat{\alpha}}\triangleq\implies$
- otherwise $\xRightarrow{\hat{\alpha}}\triangleq\implies\xrightarrow{\alpha}\implies$

S is a weak simulation if and only if $$\forall(p, q) \in S \space \forall p \xrightarrow{\alpha} p' \exists q' \space s.t. \space q\xRightarrow{\hat{\alpha}}q' \space and \space (p', q') \in S$$
A relation S is called weak bisimulation if both $S$ and $S^{-1}$ are weak simulations.
We say that p and q are weakly bisimilar, written $p \approx q$, if there exists a weak bisimulation $S$ such that $(p, q) \in S$.

**Prop:**
$\approx$ is a
1. equivalence
2. congruence
3. weak bisimulation
4. $\sim \subset \approx$

#### Examples of weakly bisimilar processes
![](../../Pasted%20image%2020250428083727.png)

**Theorem:** given any process P and any sum M, N, then:
1. $P \approx \tau.{P}$
2. $M+N+\tau.N \approx M + \tau.N$
3. $M+\alpha.P+\alpha.(N+\tau.P) \approx M + \alpha.(N + \tau.P)$

*Proof:*
take the symmetric closure of the following relations, that can be easily shown to be weak simulations:
1. $S = \{ (P,\tau.P )\}\cup Id$
2. $S=\{ (M+N+\tau.N,M+\tau.N )\}\cup Id$
3. $S=\{ ((M+\alpha.P+\alpha.(N+\tau.P), M+\alpha.(N+\tau.P)) \} \cup Id$

#### Weak bisimilarity abstracts from any $\tau$
![](../../Pasted%20image%2020250428084340.png)

**There exists no weak bisimulation S that contains (P, Q).**
*Proof:*
By contr. suppose that a bisimulation exists
Since Q −τ→ b.0, there must exist a P’ such that P ⇒ P and (P,b.0) ∈ S The only P that satisfies P ⇒ P’ is P itself
hence it should be (P,b.0) ∈ S

Contradiction: P can perform a whereas b.0 cannot !!
Similarly, P/R and Q/R are NOT weakly bisimilar

### EXAMPLE: Factory
A factory can handle three kinds of works: easy (E), medium (M), difficult (D).

An activity of the factory consists in receiving in input a work (of any kind) and in producing in output a manufactured work.

The given specification of an activity is the following:
$$A\triangleq i_{E}.A'+i_{M}. A'+i_{D}.A'$$
$$A' \triangleq \bar{o}.A$$
where actions $i_{E}, i_{M}, i_{D}$ represents they input, and $\bar{o}$ represents the production of an output.

The factory is given by the parallel composition of two activities:
$$Factory \triangleq A|A$$

A possible implementation of this specification is obtained by having two workers that perform in parallel different kinds of work.
- For easy works, they don’t use any machinery
- For medium works, they can use either a special or a general machine
- For difficult works, they have to use the special machine.
There is only one special and only one general machine that the workers have to share.

![](../../Pasted%20image%2020250428085410.png)

where rg and rs are used to require the general/special machine, lg and ls are used to leave the general/special machine, and S and G implement a semaphore on the two different machines.

The resulting system is given by:
$$Workers \triangleq (W|W|G|S) \setminus_{\{rg,rs,lg,ls  \}}$$
We now want to show that:
$$Factory \approx Workers$$
i.e., that the specification and the implementation of the factory behave the same (apart from internal actions).

Let N denote {rg,rs,lg,ls} and x,y ∊ {E,M,D}
We can prove that the following relation is a weak bisimulation:
![](../../Pasted%20image%2020250428085837.png)

This is a family of relations:
- 3 pairs of the second form (one for every x)
- 9 pairs of the fifth form (one for every x and y)
- 3 pairs of the sixth form
- 3 pairs of the seventh form

Furthermore, we should also consider commutativity of parallel in pairs of the second, third, fourth, sixth, seventh and eighth form.

Thus, R is actually made up of 1+6+2+2+9+6+6+2 = 34 pairs.
 *exercise: write down the LTSs*

### EXAMPLE: Lottery
We want to model a lottery L where we can select any ball from a bag that contains n balls; after every extraction, the extracted ball is put back in the bag and the procedure is repeated.

The specification is: $$L \triangleq \tau.\bar{p_{1}}L+\tau.\bar{p_{2}}.L+\dots+\tau.\bar{p_{n}}.L$$
where $\tau$'s represent ball extractions and $\tilde{p_{i}}$ is the action that communicates with the value of the extracted ball. The LTS resulting from this specification is:
![](../../Pasted%20image%2020250428175449.png)