Certification Problem

Input (COPS 1119)

We consider two TRSs R and S where R contains the rules

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

and S contains the following rules:

f(h(a,c)) h(f(b),h(c,h(c,h(f(h(b,b)),a)))) (4)
h(h(h(f(b),c),b),h(a,h(a,c))) h(f(b),h(c,b)) (5)
a h(h(b,b),b) (6)
c a (7)

The underlying signature is as follows:

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

Property / Task

Prove or disprove commutation.

Answer / Result

No.

Proof (by ACP @ CoCo 2023)

1 Non-Joinable Fork

The systems are not commuting due to the following forking derivations.
t0 = f(h(a,a))
S f(h(h(h(b,b),b),a))
= s1
and
t0 = f(h(a,a))
R f(b)
= t1
There is no possibility to join s1R*·←S* t1 for the following reason: