We consider the system Applicative_first_order_05__hydra.

  Alphabet:

    0 : [] --> a 
    cons : [c * c] --> c 
    copy : [a * c * c] --> c 
    f : [c] --> c 
    false : [] --> b 
    filter : [c -> b * c] --> c 
    filter2 : [b * c -> b * c * c] --> c 
    map : [c -> c * c] --> c 
    n : [] --> a 
    nil : [] --> c 
    s : [a] --> a 
    true : [] --> b 

  Rules:

    f(cons(nil, x)) => x 
    f(cons(f(cons(nil, x)), y)) => copy(n, x, y) 
    copy(0, x, y) => f(y) 
    copy(s(x), y, z) => copy(x, y, cons(f(y), z)) 
    map(g, nil) => nil 
    map(g, cons(x, y)) => cons(g x, map(g, y)) 
    filter(g, nil) => nil 
    filter(g, cons(x, y)) => filter2(g x, g, x, y) 
    filter2(true, g, x, y) => cons(x, filter(g, y)) 
    filter2(false, g, x, y) => filter(g, 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:

  f(cons(nil, X)) => X 
  f(cons(f(cons(nil, X)), Y)) => copy(n, X, Y) 
  copy(0, X, Y) => f(Y) 
  copy(s(X), Y, Z) => copy(X, Y, cons(f(Y), Z)) 

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 nattprover, this system is indeed Ce-terminating:

 || Input TRS:
 ||     1: f(cons(nil(),PeRCenTX)) -> PeRCenTX
 ||     2: f(cons(f(cons(nil(),PeRCenTX)),PeRCenTY)) -> copy(n(),PeRCenTX,PeRCenTY)
 ||     3: copy(0(),PeRCenTX,PeRCenTY) -> f(PeRCenTY)
 ||     4: copy(s(PeRCenTX),PeRCenTY,PeRCenTZ) -> copy(PeRCenTX,PeRCenTY,cons(f(PeRCenTY),PeRCenTZ))
 ||     5: TIlDePAIR(PeRCenTX,PeRCenTY) -> PeRCenTX
 ||     6: TIlDePAIR(PeRCenTX,PeRCenTY) -> PeRCenTY
 || Number of strict rules: 6
 || Direct POLO(bPol) ... failed.
 || Uncurrying copy
 || 1: f(cons(nil(),PeRCenTX)) -> PeRCenTX
 || 2: f(cons(f(cons(nil(),PeRCenTX)),PeRCenTY)) -> copy^1_n(PeRCenTX,PeRCenTY)
 || 3: copy^1_0(PeRCenTX,PeRCenTY) -> f(PeRCenTY)
 || 4: copy^1_s(PeRCenTX,PeRCenTY,PeRCenTZ) -> copy(PeRCenTX,PeRCenTY,cons(f(PeRCenTY),PeRCenTZ))
 || 5: TIlDePAIR(PeRCenTX,PeRCenTY) -> PeRCenTX
 || 6: TIlDePAIR(PeRCenTX,PeRCenTY) -> PeRCenTY
 || 7: copy(0(),_2,_3) ->= copy^1_0(_2,_3)
 || 8: copy(s(_1),_3,_4) ->= copy^1_s(_1,_3,_4)
 || 9: copy(n(),_2,_3) ->= copy^1_n(_2,_3)
 || Number of strict rules: 6
 || Direct POLO(bPol) ... failed.
 || Dependency Pairs:
 ||    #1: #copy(0(),_2,_3) ->? #copy^1_0(_2,_3)
 ||    #2: #copy^1_0(PeRCenTX,PeRCenTY) -> #f(PeRCenTY)
 ||    #3: #copy(s(_1),_3,_4) ->? #copy^1_s(_1,_3,_4)
 ||    #4: #copy^1_s(PeRCenTX,PeRCenTY,PeRCenTZ) -> #copy(PeRCenTX,PeRCenTY,cons(f(PeRCenTY),PeRCenTZ))
 ||    #5: #copy^1_s(PeRCenTX,PeRCenTY,PeRCenTZ) -> #f(PeRCenTY)
 || Number of SCCs: 1, DPs: 2
 ||   SCC { #3 #4 }
 || POLO(Sum)... succeeded.
 ||       TIlDePAIR	w: 0
 ||       copy^1_0	w: 0
 ||       s 	w: x1 + 3
 ||       n 	w: 0
 ||       #copy	w: x1 + x2 + x3
 ||       f 	w: 1
 ||       0 	w: 0
 ||       nil	w: 1
 ||       copy^1_s	w: 0
 ||       #TIlDePAIR	w: 0
 ||       copy^1_n	w: 0
 ||       #f 	w: 0
 ||       #copy^1_s	w: x1 + x2 + 2
 ||       cons	w: 1
 ||       copy	w: 0
 ||       #copy^1_0	w: 0
 ||     USABLE RULES: { }
 ||     Removed DPs: #3 #4
 || Number of SCCs: 0, DPs: 0
 || 
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:

    f(cons(nil, X)) => X 
    f(cons(f(cons(nil, X)), Y)) => copy(n, X, Y) 
    copy(0, X, Y) => f(Y) 
    copy(s(X), Y, Z) => copy(X, Y, cons(f(Y), Z)) 
    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.