Certification Problem
Input (COPS 557)
We consider the TRS containing the following rules:
| a |
→ |
b |
(1) |
|
f(x,a) |
→ |
f(b,b) |
(2) |
|
f(b,x) |
→ |
f(b,b) |
(3) |
|
f(f(x,y),z) |
→ |
f(b,b) |
(4) |
The underlying signature is as follows:
{a/0, b/0, f/2}Property / Task
Prove or disprove confluence.Answer / Result
No.Proof (by csi @ CoCo 2021)
1 Non-Joinable Fork
The system is not confluent due to the following forking derivations.
| t0
|
= |
f(f3,a) |
|
→
|
f(f3,b) |
|
= |
t1
|
| t0
|
= |
f(f3,a) |
|
→
|
f(b,b) |
|
= |
t1
|
The two resulting terms cannot be joined for the following reason:
-
The reachable terms of these two terms are approximated via the following two tree automata,
and the tree automata have an empty intersection.
-
Automaton 1
-
final states:
{9}
-
transitions:
|
f(11,10) |
→ |
9 |
| b |
→ |
10 |
| f3 |
→ |
11 |
The automaton is closed under rewriting as it is compatible.
-
Automaton 2
-
final states:
{6}
-
transitions:
The automaton is closed under rewriting as it is compatible.