Paper 3, Section II, 12H

Automata and Formal Languages
Part II, 2019

(a) State the smns-m-n theorem and the recursion theorem.

(b) State and prove Rice's theorem.

(c) Show that if g:N02N0g: \mathbb{N}_{0}^{2} \rightarrow \mathbb{N}_{0} is partial recursive, then there is some eN0e \in \mathbb{N}_{0} such that

fe,1(y)=g(e,y)yN0f_{e, 1}(y)=g(e, y) \quad \forall y \in \mathbb{N}_{0}

(d) Show there exists some mN0m \in \mathbb{N}_{0} such that WmW_{m} has exactly m2m^{2} elements.

(e) Given nN0n \in \mathbb{N}_{0}, is it possible to compute whether or not the number of elements of WnW_{n} is a (finite) perfect square? Justify your answer.

[In this question N0\mathbb{N}_{0} denotes the set of non-negative integers. Any use of Church's thesis in your answers should be explicitly stated.]