We consider the following Problem:

  Strict Trs:
    {  active(f(X, X)) -> mark(f(a(), b()))
     , active(b()) -> mark(a())
     , mark(f(X1, X2)) -> active(f(mark(X1), X2))
     , mark(a()) -> active(a())
     , mark(b()) -> active(b())
     , f(mark(X1), X2) -> f(X1, X2)
     , f(X1, mark(X2)) -> f(X1, X2)
     , f(active(X1), X2) -> f(X1, X2)
     , f(X1, active(X2)) -> f(X1, X2)}
  StartTerms: basic terms
  Strategy: innermost

Certificate: YES(?,O(n^1))

Proof:
  We consider the following Problem:
  
    Strict Trs:
      {  active(f(X, X)) -> mark(f(a(), b()))
       , active(b()) -> mark(a())
       , mark(f(X1, X2)) -> active(f(mark(X1), X2))
       , mark(a()) -> active(a())
       , mark(b()) -> active(b())
       , f(mark(X1), X2) -> f(X1, X2)
       , f(X1, mark(X2)) -> f(X1, X2)
       , f(active(X1), X2) -> f(X1, X2)
       , f(X1, active(X2)) -> f(X1, X2)}
    StartTerms: basic terms
    Strategy: innermost
  
  Certificate: YES(?,O(n^1))
  
  Proof:
    The weightgap principle applies, where following rules are oriented strictly:
    
    TRS Component:
      {  f(mark(X1), X2) -> f(X1, X2)
       , f(active(X1), X2) -> f(X1, X2)}
    
    Interpretation of nonconstant growth:
    -------------------------------------
      The following argument positions are usable:
        Uargs(active) = {1}, Uargs(f) = {1}, Uargs(mark) = {}
      We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
      Interpretation Functions:
       active(x1) = [1 0] x1 + [1]
                    [0 0]      [1]
       f(x1, x2) = [1 0] x1 + [0 0] x2 + [0]
                   [0 0]      [0 0]      [1]
       mark(x1) = [1 0] x1 + [1]
                  [0 0]      [1]
       a() = [0]
             [0]
       b() = [0]
             [0]
    
    The strictly oriented rules are moved into the weak component.
    
    We consider the following Problem:
    
      Strict Trs:
        {  active(f(X, X)) -> mark(f(a(), b()))
         , active(b()) -> mark(a())
         , mark(f(X1, X2)) -> active(f(mark(X1), X2))
         , mark(a()) -> active(a())
         , mark(b()) -> active(b())
         , f(X1, mark(X2)) -> f(X1, X2)
         , f(X1, active(X2)) -> f(X1, X2)}
      Weak Trs:
        {  f(mark(X1), X2) -> f(X1, X2)
         , f(active(X1), X2) -> f(X1, X2)}
      StartTerms: basic terms
      Strategy: innermost
    
    Certificate: YES(?,O(n^1))
    
    Proof:
      The weightgap principle applies, where following rules are oriented strictly:
      
      TRS Component: {active(b()) -> mark(a())}
      
      Interpretation of nonconstant growth:
      -------------------------------------
        The following argument positions are usable:
          Uargs(active) = {1}, Uargs(f) = {1}, Uargs(mark) = {}
        We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
        Interpretation Functions:
         active(x1) = [1 0] x1 + [1]
                      [0 0]      [1]
         f(x1, x2) = [1 0] x1 + [0 0] x2 + [0]
                     [0 0]      [0 0]      [1]
         mark(x1) = [1 0] x1 + [1]
                    [0 0]      [1]
         a() = [0]
               [0]
         b() = [2]
               [0]
      
      The strictly oriented rules are moved into the weak component.
      
      We consider the following Problem:
      
        Strict Trs:
          {  active(f(X, X)) -> mark(f(a(), b()))
           , mark(f(X1, X2)) -> active(f(mark(X1), X2))
           , mark(a()) -> active(a())
           , mark(b()) -> active(b())
           , f(X1, mark(X2)) -> f(X1, X2)
           , f(X1, active(X2)) -> f(X1, X2)}
        Weak Trs:
          {  active(b()) -> mark(a())
           , f(mark(X1), X2) -> f(X1, X2)
           , f(active(X1), X2) -> f(X1, X2)}
        StartTerms: basic terms
        Strategy: innermost
      
      Certificate: YES(?,O(n^1))
      
      Proof:
        The weightgap principle applies, where following rules are oriented strictly:
        
        TRS Component:
          {  mark(a()) -> active(a())
           , mark(b()) -> active(b())}
        
        Interpretation of nonconstant growth:
        -------------------------------------
          The following argument positions are usable:
            Uargs(active) = {1}, Uargs(f) = {1}, Uargs(mark) = {}
          We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
          Interpretation Functions:
           active(x1) = [1 0] x1 + [1]
                        [0 0]      [1]
           f(x1, x2) = [1 0] x1 + [0 0] x2 + [0]
                       [0 0]      [0 0]      [1]
           mark(x1) = [1 0] x1 + [3]
                      [0 0]      [1]
           a() = [0]
                 [0]
           b() = [2]
                 [0]
        
        The strictly oriented rules are moved into the weak component.
        
        We consider the following Problem:
        
          Strict Trs:
            {  active(f(X, X)) -> mark(f(a(), b()))
             , mark(f(X1, X2)) -> active(f(mark(X1), X2))
             , f(X1, mark(X2)) -> f(X1, X2)
             , f(X1, active(X2)) -> f(X1, X2)}
          Weak Trs:
            {  mark(a()) -> active(a())
             , mark(b()) -> active(b())
             , active(b()) -> mark(a())
             , f(mark(X1), X2) -> f(X1, X2)
             , f(active(X1), X2) -> f(X1, X2)}
          StartTerms: basic terms
          Strategy: innermost
        
        Certificate: YES(?,O(n^1))
        
        Proof:
          The weightgap principle applies, where following rules are oriented strictly:
          
          TRS Component: {f(X1, mark(X2)) -> f(X1, X2)}
          
          Interpretation of nonconstant growth:
          -------------------------------------
            The following argument positions are usable:
              Uargs(active) = {1}, Uargs(f) = {1}, Uargs(mark) = {}
            We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
            Interpretation Functions:
             active(x1) = [1 0] x1 + [0]
                          [0 1]      [0]
             f(x1, x2) = [1 0] x1 + [1 0] x2 + [0]
                         [0 0]      [0 0]      [1]
             mark(x1) = [1 0] x1 + [1]
                        [0 0]      [1]
             a() = [0]
                   [0]
             b() = [2]
                   [1]
          
          The strictly oriented rules are moved into the weak component.
          
          We consider the following Problem:
          
            Strict Trs:
              {  active(f(X, X)) -> mark(f(a(), b()))
               , mark(f(X1, X2)) -> active(f(mark(X1), X2))
               , f(X1, active(X2)) -> f(X1, X2)}
            Weak Trs:
              {  f(X1, mark(X2)) -> f(X1, X2)
               , mark(a()) -> active(a())
               , mark(b()) -> active(b())
               , active(b()) -> mark(a())
               , f(mark(X1), X2) -> f(X1, X2)
               , f(active(X1), X2) -> f(X1, X2)}
            StartTerms: basic terms
            Strategy: innermost
          
          Certificate: YES(?,O(n^1))
          
          Proof:
            The weightgap principle applies, where following rules are oriented strictly:
            
            TRS Component: {f(X1, active(X2)) -> f(X1, X2)}
            
            Interpretation of nonconstant growth:
            -------------------------------------
              The following argument positions are usable:
                Uargs(active) = {1}, Uargs(f) = {1}, Uargs(mark) = {}
              We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
              Interpretation Functions:
               active(x1) = [1 0] x1 + [1]
                            [0 1]      [0]
               f(x1, x2) = [1 0] x1 + [1 0] x2 + [0]
                           [0 0]      [0 0]      [1]
               mark(x1) = [1 0] x1 + [1]
                          [0 0]      [1]
               a() = [0]
                     [0]
               b() = [0]
                     [1]
            
            The strictly oriented rules are moved into the weak component.
            
            We consider the following Problem:
            
              Strict Trs:
                {  active(f(X, X)) -> mark(f(a(), b()))
                 , mark(f(X1, X2)) -> active(f(mark(X1), X2))}
              Weak Trs:
                {  f(X1, active(X2)) -> f(X1, X2)
                 , f(X1, mark(X2)) -> f(X1, X2)
                 , mark(a()) -> active(a())
                 , mark(b()) -> active(b())
                 , active(b()) -> mark(a())
                 , f(mark(X1), X2) -> f(X1, X2)
                 , f(active(X1), X2) -> f(X1, X2)}
              StartTerms: basic terms
              Strategy: innermost
            
            Certificate: YES(?,O(n^1))
            
            Proof:
              The weightgap principle applies, where following rules are oriented strictly:
              
              TRS Component: {active(f(X, X)) -> mark(f(a(), b()))}
              
              Interpretation of nonconstant growth:
              -------------------------------------
                The following argument positions are usable:
                  Uargs(active) = {1}, Uargs(f) = {1}, Uargs(mark) = {}
                We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
                Interpretation Functions:
                 active(x1) = [1 2] x1 + [0]
                              [0 0]      [0]
                 f(x1, x2) = [1 0] x1 + [0 0] x2 + [0]
                             [0 0]      [0 0]      [2]
                 mark(x1) = [1 0] x1 + [0]
                            [0 0]      [0]
                 a() = [0]
                       [0]
                 b() = [0]
                       [0]
              
              The strictly oriented rules are moved into the weak component.
              
              We consider the following Problem:
              
                Strict Trs: {mark(f(X1, X2)) -> active(f(mark(X1), X2))}
                Weak Trs:
                  {  active(f(X, X)) -> mark(f(a(), b()))
                   , f(X1, active(X2)) -> f(X1, X2)
                   , f(X1, mark(X2)) -> f(X1, X2)
                   , mark(a()) -> active(a())
                   , mark(b()) -> active(b())
                   , active(b()) -> mark(a())
                   , f(mark(X1), X2) -> f(X1, X2)
                   , f(active(X1), X2) -> f(X1, X2)}
                StartTerms: basic terms
                Strategy: innermost
              
              Certificate: YES(?,O(n^1))
              
              Proof:
                We consider the following Problem:
                
                  Strict Trs: {mark(f(X1, X2)) -> active(f(mark(X1), X2))}
                  Weak Trs:
                    {  active(f(X, X)) -> mark(f(a(), b()))
                     , f(X1, active(X2)) -> f(X1, X2)
                     , f(X1, mark(X2)) -> f(X1, X2)
                     , mark(a()) -> active(a())
                     , mark(b()) -> active(b())
                     , active(b()) -> mark(a())
                     , f(mark(X1), X2) -> f(X1, X2)
                     , f(active(X1), X2) -> f(X1, X2)}
                  StartTerms: basic terms
                  Strategy: innermost
                
                Certificate: YES(?,O(n^1))
                
                Proof:
                  The problem is match-bounded by 0.
                  The enriched problem is compatible with the following automaton:
                  {  active_0(2) -> 1
                   , f_0(2, 2) -> 1
                   , mark_0(2) -> 1
                   , a_0() -> 2
                   , b_0() -> 2}

Hurray, we answered YES(?,O(n^1))