We consider the system Applicative_first_order_05__motivation.

  Alphabet:

    cons : [c * d] --> d 
    f : [a * a] --> a 
    false : [] --> b 
    filter : [c -> b * d] --> d 
    filter2 : [b * c -> b * c * d] --> d 
    g : [a] --> a 
    h : [a] --> a 
    map : [c -> c * d] --> d 
    nil : [] --> d 
    true : [] --> b 

  Rules:

    g(h(g(x))) => g(x) 
    g(g(x)) => g(h(g(x))) 
    h(h(x)) => h(f(h(x), x)) 
    map(i, nil) => nil 
    map(i, cons(x, y)) => cons(i x, map(i, y)) 
    filter(i, nil) => nil 
    filter(i, cons(x, y)) => filter2(i x, i, x, y) 
    filter2(true, i, x, y) => cons(x, filter(i, y)) 
    filter2(false, i, x, y) => filter(i, y) 

This AFS is converted to an AFSM simply by replacing all free variables by meta-variables (with arity 0).

We observe that the rules contain a first-order subset:

  g(h(g(X))) => g(X) 
  g(g(X)) => g(h(g(X))) 
  h(h(X)) => h(f(h(X), X)) 

Moreover, the system is finitely branching.  Thus, by [Kop12, Thm. 7.55], we may omit all first-order dependency pairs from the dependency pair problem (DP(R), R) if this first-order part is Ce-terminating when seen as a many-sorted first-order TRS.

According to mutermprover, this system is indeed Ce-terminating:

 || 
 || Problem 1: 
 || 
 || (VAR %X %Y)
 || (RULES
 || g(g(%X)) -> g(h(g(%X)))
 || g(h(g(%X))) -> g(%X)
 || h(h(%X)) -> h(f(h(%X),%X))
 || ~PAIR(%X,%Y) -> %X
 || ~PAIR(%X,%Y) -> %Y
 || )
 || 
 || Problem 1: 
 || 
 || Dependency Pairs Processor:
 || -> Pairs:
 ||  G(g(%X)) -> G(h(g(%X)))
 ||  G(g(%X)) -> H(g(%X))
 ||  H(h(%X)) -> H(f(h(%X),%X))
 || -> Rules:
 ||  g(g(%X)) -> g(h(g(%X)))
 ||  g(h(g(%X))) -> g(%X)
 ||  h(h(%X)) -> h(f(h(%X),%X))
 ||  ~PAIR(%X,%Y) -> %X
 ||  ~PAIR(%X,%Y) -> %Y
 || 
 || Problem 1: 
 || 
 || SCC Processor:
 || -> Pairs:
 ||  G(g(%X)) -> G(h(g(%X)))
 ||  G(g(%X)) -> H(g(%X))
 ||  H(h(%X)) -> H(f(h(%X),%X))
 || -> Rules:
 ||  g(g(%X)) -> g(h(g(%X)))
 ||  g(h(g(%X))) -> g(%X)
 ||  h(h(%X)) -> h(f(h(%X),%X))
 ||  ~PAIR(%X,%Y) -> %X
 ||  ~PAIR(%X,%Y) -> %Y
 || ->Strongly Connected Components:
 || ->->Cycle:
 || ->->-> Pairs:
 ||  G(g(%X)) -> G(h(g(%X)))
 || ->->-> Rules:
 ||  g(g(%X)) -> g(h(g(%X)))
 ||  g(h(g(%X))) -> g(%X)
 ||  h(h(%X)) -> h(f(h(%X),%X))
 ||  ~PAIR(%X,%Y) -> %X
 ||  ~PAIR(%X,%Y) -> %Y
 || 
 || Problem 1: 
 || 
 || Reduction Pair Processor:
 || -> Pairs:
 ||  G(g(%X)) -> G(h(g(%X)))
 || -> Rules:
 ||  g(g(%X)) -> g(h(g(%X)))
 ||  g(h(g(%X))) -> g(%X)
 ||  h(h(%X)) -> h(f(h(%X),%X))
 ||  ~PAIR(%X,%Y) -> %X
 ||  ~PAIR(%X,%Y) -> %Y
 || -> Usable rules:
 ||  g(g(%X)) -> g(h(g(%X)))
 ||  g(h(g(%X))) -> g(%X)
 ||  h(h(%X)) -> h(f(h(%X),%X))
 || ->Interpretation type:
 ||  Linear
 || ->Coefficients:
 ||  Natural Numbers
 || ->Dimension:
 ||  1
 || ->Bound:
 ||  2
 || ->Interpretation:
 ||  
 || [g](X) = 1
 || [h](X) = 0
 || [f](X1,X2) = 2.X1 + 2.X2
 || [G](X) = 2.X
 || 
 || Problem 1: 
 || 
 || SCC Processor:
 || -> Pairs:
 ||  Empty
 || -> Rules:
 ||  g(g(%X)) -> g(h(g(%X)))
 ||  g(h(g(%X))) -> g(%X)
 ||  h(h(%X)) -> h(f(h(%X),%X))
 ||  ~PAIR(%X,%Y) -> %X
 ||  ~PAIR(%X,%Y) -> %Y
 || ->Strongly Connected Components:
 ||  There is no strongly connected component
 || 
 || The problem is finite.
 || 
We use the dependency pair framework as described in [Kop12, Ch. 6/7], with static dependency pairs (see [KusIsoSakBla09] and the adaptation for AFSMs and accessible arguments in [FuhKop19]).

We thus obtain the following dependency pair problem (P_0, R_0, computable, formative):

  Dependency Pairs P_0:

    0] map#(F, cons(X, Y)) =#> map#(F, Y)   
    1] filter#(F, cons(X, Y)) =#> filter2#(F X, F, X, Y)   
    2] filter2#(true, F, X, Y) =#> filter#(F, Y)   
    3] filter2#(false, F, X, Y) =#> filter#(F, Y)   

  Rules R_0:

    g(h(g(X))) => g(X) 
    g(g(X)) => g(h(g(X))) 
    h(h(X)) => h(f(h(X), X)) 
    map(F, nil) => nil 
    map(F, cons(X, Y)) => cons(F X, map(F, Y)) 
    filter(F, nil) => nil 
    filter(F, cons(X, Y)) => filter2(F X, F, X, Y) 
    filter2(true, F, X, Y) => cons(X, filter(F, Y)) 
    filter2(false, F, X, Y) => filter(F, Y) 

Thus, the original system is terminating if (P_0, R_0, computable, formative) is finite.

We consider the dependency pair problem (P_0, R_0, computable, formative).

We place the elements of P in a dependency graph approximation G (see e.g. [Kop12, Thm. 7.27, 7.29], as follows:

    * 0 : 0 
    * 1 : 2, 3 
    * 2 : 1 
    * 3 : 1 

This graph has the following strongly connected components:

  P_1:

    map#(F, cons(X, Y)) =#> map#(F, Y)   

  P_2:

    filter#(F, cons(X, Y)) =#> filter2#(F X, F, X, Y)   
    filter2#(true, F, X, Y) =#> filter#(F, Y)   
    filter2#(false, F, X, Y) =#> filter#(F, Y)   

By [Kop12, Thm. 7.31], we may replace any dependency pair problem (P_0, R_0, m, f) by (P_1, R_0, m, f) and (P_2, R_0, m, f).

Thus, the original system is terminating if each of (P_1, R_0, computable, formative) and (P_2, R_0, computable, formative) is finite.

We consider the dependency pair problem (P_2, R_0, computable, formative).

We apply the subterm criterion with the following projection function:

  nu(filter2#) = 4 
  nu(filter#) = 2 

Thus, we can orient the dependency pairs as follows:

  nu(filter#(F, cons(X, Y))) = cons(X, Y) |> Y = nu(filter2#(F X, F, X, Y)) 
  nu(filter2#(true, F, X, Y)) = Y = Y = nu(filter#(F, Y)) 
  nu(filter2#(false, F, X, Y)) = Y = Y = nu(filter#(F, Y)) 

By [FuhKop19, Thm. 61], we may replace a dependency pair problem (P_2, R_0, computable, f) by (P_3, R_0, computable, f), where P_3 contains:

  filter2#(true, F, X, Y) =#> filter#(F, Y)   
  filter2#(false, F, X, Y) =#> filter#(F, Y)   

Thus, the original system is terminating if each of (P_1, R_0, computable, formative) and (P_3, R_0, computable, formative) is finite.

We consider the dependency pair problem (P_3, R_0, computable, formative).

We place the elements of P in a dependency graph approximation G (see e.g. [Kop12, Thm. 7.27, 7.29], as follows:

    * 0 :  
    * 1 :  

This graph has no strongly connected components.  By [Kop12, Thm. 7.31], this implies finiteness of the dependency pair problem.

Thus, the original system is terminating if (P_1, R_0, computable, formative) is finite.

We consider the dependency pair problem (P_1, R_0, computable, formative).

We apply the subterm criterion with the following projection function:

  nu(map#) = 2 

Thus, we can orient the dependency pairs as follows:

  nu(map#(F, cons(X, Y))) = cons(X, Y) |> Y = nu(map#(F, Y)) 

By [FuhKop19, Thm. 61], we may replace a dependency pair problem (P_1, R_0, computable, f) by ({}, R_0, computable, f).  By the empty set processor [Kop12, Thm. 7.15] this problem may be immediately removed.

As all dependency pair problems were succesfully simplified with sound (and complete) processors until nothing remained, we conclude termination.


+++ Citations +++

[FuhKop19]  C. Fuhs, and C. Kop.  A static higher-order dependency pair framework.  In Proceedings of ESOP 2019, 2019.
[Kop12]  C. Kop.  Higher Order Termination.  PhD Thesis, 2012.
[KusIsoSakBla09]  K. Kusakari, Y. Isogai, M. Sakai, and F. Blanqui.  Static Dependency Pair Method Based On Strong Computability for Higher-Order Rewrite Systems.  In volume 92(10) of IEICE Transactions on Information and Systems.  2007--2015, 2009.