We consider the system 445.

  Alphabet:

    f : [(A -> A) -> A] --> A 

  Rules:

    f(/\g.X(g)) => X(/\x.x) 

We use the dependency pair framework as described in [Kop12, Ch. 6/7], with dynamic dependency pairs.

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

  Dependency Pairs P_0:

    0] f#(/\g.X(g)) =#> X(/\x.x)   

  Rules R_0:

    f(/\g.X(g)) => X(/\x.x) 

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

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

This combination (P_0, R_0) has no formative rules!  We will name the empty set of rules:R_1.

By [Kop12, Thm. 7.17], we may replace the dependency pair problem (P_0, R_0, minimal, formative) by (P_0, R_1, minimal, formative).

Thus, the original system is terminating if (P_0, R_1, minimal, formative) is finite.

We consider the dependency pair problem (P_0, R_1, minimal, formative).

We will use the reduction pair processor [Kop12, Thm. 7.16].  As the system is abstraction-simple and the formative flag is set, it suffices to find a tagged reduction pair [Kop12, Def. 6.70].  Thus, we must orient:

  f#(/\g.X(g)) >? X(/\x.x) 

We apply [Kop12, Thm. 6.75] and use the following argument functions:

  pi( f#(F) ) = #argfun-f##(F (/\x.x)) 

We orient these requirements with a polynomial interpretation in the natural numbers.

The following interpretation satisfies the requirements:

  #argfun-f## = \y0.1 + y0 
  f# = \G0.0 

Using this interpretation, the requirements translate to:

  [[#argfun-f##((/\g._x0(g)) (/\x.x))]] = 1 + F0(\y0.y0) > F0(\y0.y0) = [[_x0(/\x.x)]] 

By the observations in [Kop12, Sec. 6.6], this reduction pair suffices; we may thus replace a dependency pair problem (P_0, R_1) by ({}, R_1).  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 +++

[Kop12]  C. Kop.  Higher Order Termination.  PhD Thesis, 2012.