Certification Problem
Input (COPS 580)
We consider the TRS containing the following rules:
|
+(0,0) |
→ |
0 |
(1) |
|
+(s(0),y) |
→ |
s(+(0,y)) |
(2) |
|
+(x,s(y)) |
→ |
s(+(y,x)) |
(3) |
|
s(s(x)) |
→ |
x |
(4) |
The underlying signature is as follows:
{+/2, 0/0, s/1}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
|
= |
+(x,s(+(s(0),x20))) |
|
→
|
+(x,s(s(+(0,x20)))) |
|
→
|
+(x,+(0,x20)) |
|
= |
t2
|
| t0
|
= |
+(x,s(+(s(0),x20))) |
|
→
|
s(+(+(s(0),x20),x)) |
|
→
|
s(+(s(+(0,x20)),x)) |
|
= |
t2
|
The two resulting terms cannot be joined for the following reason:
- When applying the cap-function on both terms (where variables may be treated like constants)
then the resulting terms do not unify.