Certification Problem

Input (COPS 698)

We consider the TRS containing the following rules:

c	→	f(h(b,b))	(1)
c	→	b	(2)
b	→	b	(3)
c	→	a	(4)

The underlying signature is as follows:

{c/0, f/1, h/2, b/0, a/0}

Property / Task

Prove or disprove confluence.

Answer / Result

No.

Proof (by csi @ CoCo 2023)

1 Non-Joinable Fork

The system is not confluent due to the following forking derivations.

t₀	=	c
	→	a
	=	t₁

t₀	=	c
	→	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:
    
    {1}
  - transitions:
    
    a → 1
  The automaton is closed under rewriting as it is compatible.
- Automaton 2
  - final states:
    
    {2}
  - transitions:
    
    b → 2
  The automaton is closed under rewriting as it is compatible.