Certification Problem

Input (COPS 678)

We consider the TRS containing the following rules:

b	→	a	(1)
b	→	c	(2)
b	→	h(h(c,a),f(f(b)))	(3)
a	→	f(a)	(4)
b	→	h(a,a)	(5)

The underlying signature is as follows:

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

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	=	b
	→	a
	=	t₁

t₀	=	b
	→	c
	=	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:
    
    {12}
  - transitions:
    
    f(12) → 12
    
    a → 12
  The automaton is closed under rewriting as it is compatible.
- Automaton 2
  - final states:
    
    {11}
  - transitions:
    
    c → 11
  The automaton is closed under rewriting as it is compatible.