English

An anomaly in diagonalization

Logic 2025-04-21 v9

Abstract

When formalized, some diagonal arguments do not show the diagonal object to be impossible but rather reveal some other anomaly (e.g., that one of the relevant sets is ill-defined). This raises the possibility that some diagonal arguments have been misinterpreted along this parameter. The diagonal argument against a universal p.r. function is considered in this light. The impetus is the construction of a binary p.r. function that apparently computes, for any ii and nn, fi(i,n)f_i\left(i,n\right). The construction features an algorithm which exploits that, in the theory of concern, the index assigned to a p.r. function codes the definitional composition of the function. The algorithm is guided by this to generate a "canonical proof" of fi(i,n)=mf_i\left(i,n\right)=m, and a dynamically updated counter tracks how many computations are needed before halting. The resulting algorithm and function then appear to satisfy all standard criteria for being p.r. while simulating a universal function. This suggests that the standard diagonal argument does not apply straightforwardly in this setting, despite surface-level compliance with its assumptions. The case points to a need for greater clarity about these assumptions.

Keywords

Cite

@article{arxiv.2103.02489,
  title  = {An anomaly in diagonalization},
  author = {T. Parent},
  journal= {arXiv preprint arXiv:2103.02489},
  year   = {2025}
}
R2 v1 2026-06-23T23:42:59.389Z