YES

We show the termination of the TRS R:

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

-- SCC decomposition.

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

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

and R consists of:

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

The estimated dependency graph contains the following SCCs:

  {p2}


-- Reduction pair.

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

p1: f#(g(x)) -> f#(x)

and R consists of:

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

The set of usable rules consists of

  (no rules)

Take the monotone reduction pair:

  lexicographic combination of reduction pairs:
  
    1. matrix interpretations:
    
      carrier: N^1
      order: standard order
      interpretations:
        f#_A(x1) = x1
        g_A(x1) = x1 + 1
    
    2. lexicographic path order with precedence:
    
      precedence:
      
        g > f#
      
      argument filter:
    
        pi(f#) = 1
        pi(g) = [1]
    

The next rules are strictly ordered:

  p1
  r1

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