Certification Problem
Input (COPS 13)
We consider the TRS containing the following rules:
|
f(f(x,y),z) |
→ |
f(x,f(y,z)) |
(1) |
|
f(x,1) |
→ |
x |
(2) |
The underlying signature is as follows:
{f/2, 1/0}Property / Task
Prove or disprove confluence.Answer / Result
No.Proof (by csi @ CoCo 2022)
1 Non-Joinable Fork
The system is not confluent due to the following forking derivations.
| t0
|
= |
f(f(f8,1),f5) |
|
→
|
f(f8,f5) |
|
= |
t1
|
| t0
|
= |
f(f(f8,1),f5) |
|
→
|
f(f8,f(1,f5)) |
|
= |
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:
{1}
-
transitions:
The automaton is closed under rewriting as it is compatible.
-
Automaton 2
-
final states:
{4}
-
transitions:
| f5 |
→ |
5 |
| f8 |
→ |
8 |
| 1 |
→ |
6 |
|
f(8,7) |
→ |
4 |
|
f(6,5) |
→ |
7 |
The automaton is closed under rewriting as it is compatible.