YES

We show the termination of the TRS R:

  f(a(),f(x,a())) -> f(a(),f(x,f(f(a(),a()),a())))

-- SCC decomposition.

Consider the dependency pair problem (P, R), where P consists of

p1: f#(a(),f(x,a())) -> f#(a(),f(x,f(f(a(),a()),a())))
p2: f#(a(),f(x,a())) -> f#(x,f(f(a(),a()),a()))
p3: f#(a(),f(x,a())) -> f#(f(a(),a()),a())
p4: f#(a(),f(x,a())) -> f#(a(),a())

and R consists of:

r1: f(a(),f(x,a())) -> f(a(),f(x,f(f(a(),a()),a())))

The estimated dependency graph contains the following SCCs:

  {p1, p2}


-- Reduction pair.

Consider the dependency pair problem (P, R), where P consists of

p1: f#(a(),f(x,a())) -> f#(a(),f(x,f(f(a(),a()),a())))
p2: f#(a(),f(x,a())) -> f#(x,f(f(a(),a()),a()))

and R consists of:

r1: f(a(),f(x,a())) -> f(a(),f(x,f(f(a(),a()),a())))

The set of usable rules consists of

  r1

Take the reduction pair:

  max/plus interpretations on natural numbers:
  
    f#_A(x1,x2) = max{0, x1 - 17, x2 - 11}
    a_A = 18
    f_A(x1,x2) = max{9, x1 - 5, x2 - 8}

The next rules are strictly ordered:

  p2

We remove them from the problem.

-- SCC decomposition.

Consider the dependency pair problem (P, R), where P consists of

p1: f#(a(),f(x,a())) -> f#(a(),f(x,f(f(a(),a()),a())))

and R consists of:

r1: f(a(),f(x,a())) -> f(a(),f(x,f(f(a(),a()),a())))

The estimated dependency graph contains the following SCCs:

  {p1}


-- Reduction pair.

Consider the dependency pair problem (P, R), where P consists of

p1: f#(a(),f(x,a())) -> f#(a(),f(x,f(f(a(),a()),a())))

and R consists of:

r1: f(a(),f(x,a())) -> f(a(),f(x,f(f(a(),a()),a())))

The set of usable rules consists of

  r1

Take the reduction pair:

  max/plus interpretations on natural numbers:
  
    f#_A(x1,x2) = max{x1 - 7, x2}
    a_A = 9
    f_A(x1,x2) = max{3, x2 - 4}

The next rules are strictly ordered:

  p1

We remove them from the problem.  Then no dependency pair remains.