Certification Problem

Input (COPS 567)

We consider the TRS containing the following rules:

f(g(f(x))) g(f(g(x))) (1)
f(c) c (2)
g(c) c (3)

The underlying signature is as follows:

{f/1, g/1, c/0}

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(g(f(g(f(f3)))))
f(g(g(f(g(f3)))))
= t1

t0 = f(g(f(g(f(f3)))))
g(f(g(g(f(f3)))))
= t1

The two resulting terms cannot be joined for the following reason: