We consider the system Applicative_AG01_innermost__#4.34.

  Alphabet:

    0 : [] --> b 
    1 : [] --> b 
    c : [b] --> b 
    cons : [c * d] --> d 
    f : [b] --> a 
    false : [] --> a 
    filter : [c -> a * d] --> d 
    filter2 : [a * c -> a * c * d] --> d 
    g : [b * b] --> b 
    if : [a * b * b] --> b 
    map : [c -> c * d] --> d 
    nil : [] --> d 
    s : [b] --> b 
    true : [] --> a 

  Rules:

    f(0) => true 
    f(1) => false 
    f(s(x)) => f(x) 
    if(true, x, y) => x 
    if(false, x, y) => y 
    g(s(x), s(y)) => if(f(x), s(x), s(y)) 
    g(x, c(y)) => g(x, g(s(c(y)), y)) 
    map(h, nil) => nil 
    map(h, cons(x, y)) => cons(h x, map(h, y)) 
    filter(h, nil) => nil 
    filter(h, cons(x, y)) => filter2(h x, h, x, y) 
    filter2(true, h, x, y) => cons(x, filter(h, y)) 
    filter2(false, h, x, y) => filter(h, 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(0) => true 
  f(1) => false 
  f(s(X)) => f(X) 
  if(true, X, Y) => X 
  if(false, X, Y) => Y 
  g(s(X), s(Y)) => if(f(X), s(X), s(Y)) 
  g(X, c(Y)) => g(X, g(s(c(Y)), Y)) 

Moreover, the system is orthogonal.  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 terminating when seen as a many-sorted first-order TRS.

According to mutermprover, this system is indeed terminating:

 || 
 || Problem 1: 
 || 
 || (VAR %X %Y)
 || (RULES
 || f(0) -> true
 || f(1) -> false
 || f(s(%X)) -> f(%X)
 || g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 || g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 || if(false,%X,%Y) -> %Y
 || if(true,%X,%Y) -> %X
 || )
 || 
 || Problem 1: 
 || 
 || Innermost Equivalent Processor:
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || -> The term rewriting system is non-overlaping or locally confluent overlay system. Therefore, innermost termination implies termination.
 ||  
 || 
 || Problem 1: 
 || 
 || Dependency Pairs Processor:
 || -> Pairs:
 ||  F(s(%X)) -> F(%X)
 ||  G(s(%X),s(%Y)) -> F(%X)
 ||  G(s(%X),s(%Y)) -> IF(f(%X),s(%X),s(%Y))
 ||  G(%X,c(%Y)) -> G(s(c(%Y)),%Y)
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || 
 || Problem 1: 
 || 
 || SCC Processor:
 || -> Pairs:
 ||  F(s(%X)) -> F(%X)
 ||  G(s(%X),s(%Y)) -> F(%X)
 ||  G(s(%X),s(%Y)) -> IF(f(%X),s(%X),s(%Y))
 ||  G(%X,c(%Y)) -> G(s(c(%Y)),%Y)
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->Strongly Connected Components:
 || ->->Cycle:
 || ->->-> Pairs:
 ||  F(s(%X)) -> F(%X)
 || ->->-> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->->Cycle:
 || ->->-> Pairs:
 ||  G(%X,c(%Y)) -> G(s(c(%Y)),%Y)
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || ->->-> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || 
 || 
 || The problem is decomposed in 2 subproblems.
 || 
 || Problem 1.1: 
 || 
 || Subterm Processor:
 || -> Pairs:
 ||  F(s(%X)) -> F(%X)
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->Projection:
 ||  pi(F) = 1
 || 
 || Problem 1.1: 
 || 
 || SCC Processor:
 || -> Pairs:
 ||  Empty
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->Strongly Connected Components:
 ||  There is no strongly connected component
 || 
 || The problem is finite.
 || 
 || Problem 1.2: 
 || 
 || Reduction Pairs Processor:
 || -> Pairs:
 ||  G(%X,c(%Y)) -> G(s(c(%Y)),%Y)
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || -> Usable rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->Interpretation type:
 ||  Linear
 || ->Coefficients:
 ||  Natural Numbers
 || ->Dimension:
 ||  1
 || ->Bound:
 ||  2
 || ->Interpretation:
 ||  
 || [f](X) = 0
 || [g](X1,X2) = 2.X2 + 2
 || [if](X1,X2,X3) = 2.X1 + X2 + X3 + 2
 || [0] = 2
 || [1] = 0
 || [c](X) = 2.X + 2
 || [false] = 0
 || [s](X) = 2
 || [true] = 0
 || [G](X1,X2) = X1 + 2.X2
 || 
 || Problem 1.2: 
 || 
 || SCC Processor:
 || -> Pairs:
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->Strongly Connected Components:
 || ->->Cycle:
 || ->->-> Pairs:
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || ->->-> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || 
 || Problem 1.2: 
 || 
 || Reduction Pairs Processor:
 || -> Pairs:
 ||  G(%X,c(%Y)) -> G(%X,g(s(c(%Y)),%Y))
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || -> Usable rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->Interpretation type:
 ||  Linear
 || ->Coefficients:
 ||  Natural Numbers
 || ->Dimension:
 ||  1
 || ->Bound:
 ||  2
 || ->Interpretation:
 ||  
 || [f](X) = 0
 || [g](X1,X2) = X1 + 2.X2
 || [if](X1,X2,X3) = 2.X1 + 2.X2 + X3
 || [0] = 2
 || [1] = 2
 || [c](X) = 2.X + 2
 || [false] = 0
 || [s](X) = 1
 || [true] = 0
 || [G](X1,X2) = 2.X2
 || 
 || Problem 1.2: 
 || 
 || SCC Processor:
 || -> Pairs:
 ||  Empty
 || -> Rules:
 ||  f(0) -> true
 ||  f(1) -> false
 ||  f(s(%X)) -> f(%X)
 ||  g(s(%X),s(%Y)) -> if(f(%X),s(%X),s(%Y))
 ||  g(%X,c(%Y)) -> g(%X,g(s(c(%Y)),%Y))
 ||  if(false,%X,%Y) -> %Y
 ||  if(true,%X,%Y) -> %X
 || ->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:

    f(0) => true 
    f(1) => false 
    f(s(X)) => f(X) 
    if(true, X, Y) => X 
    if(false, X, Y) => Y 
    g(s(X), s(Y)) => if(f(X), s(X), s(Y)) 
    g(X, c(Y)) => g(X, g(s(c(Y)), Y)) 
    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.