XII. Elementary

Yuxi Fu

BASICS, Shanghai Jiao Tong University What is the class of arithmetic functions we use in ?

Computability Theory, by Y. Fu XII. Elementary Function 1 / 17 Definition

The class E of elementary function is constructed from 1. zero, successor, projection, and 2. subtraction x−˙ y (for defining conditionals); and is 3. closed under composition and 4. bounded sum/product (bounded recursion).

Remark. 1. and can be defined using bounded sum; (hyper) exponential can be defined using bounded product. 2. Lower elementary functions are constructed by dropping (4). 3. A predicate is elementary if its characterization function is elementary.

Computability Theory, by Y. Fu XII. Elementary Function 2 / 17 Bounded Minimalisation is Elementary

Fact. E is closed under bounded minimalisation. Proof. Suppose f (x, z) is elementary. Then µz < y.f (x, z) = 0 is X Y sg(f (x, u)). v

It is easy to see that sg is elementary.

Computability Theory, by Y. Fu XII. Elementary Function 3 / 17 Logical Operation

Fact. The set of elementary predicates is closed under negation, conjunction, disjunction and bounded quantifiers.

Computability Theory, by Y. Fu XII. Elementary Function 4 / 17 Basic Arithmetic Functions are Elementary

y Q 2 1. The exponential x is defined by z

2. The function px is defined by

2x px = µy < 2 .(x = 0 or y is the xth prime)   2x X = µy < 2 . x = Pr(z) z≤y   2x X = µy < 2 . x−˙ Pr(z) = 0 . z≤y

Computability Theory, by Y. Fu XII. Elementary Function 5 / 17 Bounded Recursion

Fact. Let f (x) and g(x, y, z) be elementary and h(x, y) be defined from f , g via primitive recursion. If h(x, y) ≤ b(x, y) for some elementary function b(x, y), then h(x, y) is elementary. Proof. Observe that

h(x,0) h(x,1) h(x,y) Y b(x,z) 2 3 ... py+1 ≤ pz+1 . z≤y

So we can define h(x, y) by

Y b(x,z) µe ≤ pz+1 . ((e)1 = f (x) ∧ ∀z < y.((e)z+2 = g(x, z, (e)z+1))) . z≤y

We are done.

Computability Theory, by Y. Fu XII. Elementary Function 6 / 17 G¨odelEncoding is Elementary

Fact. G¨odelencoding functions are elementary.

Computability Theory, by Y. Fu XII. Elementary Function 7 / 17 Kleene’s Predicate is Elementary

Fact. The Kleene’s functions σn, cn and jn are elementary.

Computability Theory, by Y. Fu XII. Elementary Function 8 / 17 Elementary Time Functions

A φe is in elementary time if te (x) ≤ b(x) almost everywhere for some elementary function b(x).

Fact. The elementary time functions are elementary. Proof. φe (x) is almost everywhere computable by the elementary function

(cn(e, ex, µt ≤ b(ex).jn(e, ex, t) = 0))1,

which implies that φe (x) is elementary.

Computability Theory, by Y. Fu XII. Elementary Function 9 / 17 It has been suggested that E contains practical computable functions.

Computability Theory, by Y. Fu XII. Elementary Function 10 / 17 A computable function f (x) is practically computable if it can be computed in 2x 2··· expk (x) = 2 | {z } k exp (x) steps for some k. We let 2 k stand for expk+1(x).

Computability Theory, by Y. Fu XII. Elementary Function 11 / 17 Upper Bound of Elementary Functions

Theorem. For each elementary function f (ex) there is some k such that f (ex) ≤ expk (max{ex}). Proof. The basic elementary functions satisfy the upper bound. The elementary operations preserves the upper bound.

Computability Theory, by Y. Fu XII. Elementary Function 12 / 17 Corollary. expx (x) is primitive recursive but not elementary. Proof. The function expx (x) is defined by g(x, x), where

g(x, 0) = x, g(x, y + 1) = 2g(x,y).

We are done.

Computability Theory, by Y. Fu XII. Elementary Function 13 / 17 Elementary Functions are Elementary Time

Lemma. Suppose f (ex) and g(ex, y, z) are in elementary time and h(ex, y) is defined from f , g via recursion. If h(ex, y) is elementary, then h(ex, y) is in elementary time. Proof. The standard program that calculates h does it in elementary time.

Computability Theory, by Y. Fu XII. Elementary Function 14 / 17 Elementary Functions are Elementary Time

Theorem. If f (ex) is elementary, then there is a program P for f n such that tP (ex) is elementary. Proof. Use the above lemma.

Computability Theory, by Y. Fu XII. Elementary Function 15 / 17 Complexity Theoretical Characterization

Theorem. A total function f (ex) is elementary iff it is computable in time expk (max{ex}) for some k.

Computability Theory, by Y. Fu XII. Elementary Function 16 / 17 n 2n ELEMENTARY = TIME(2n) ∪ TIME(22 ) ∪ TIME(22 ) ∪ ....

Computability Theory, by Y. Fu XII. Elementary Function 17 / 17