# [FOM] What does Peano arithmetic have to offer?

Harvey Friedman friedman at math.ohio-state.edu
Sat May 1 14:25:14 EDT 2010

```The key divide is whether one is interested in

#) *HAVING A SCIENTIFIC THEORY OF MATHEMATICAL PROOF*

as opposed to ad hoc observations. The ad hoc observations may well be
deep, insightful, exciting, useful, etcetera. And also the ad hoc
observations may not be YET amenable to scientific treatment. We
always strive to incorporated more and deeper phenomena into our
scientific theories.

Nevertheless, it was only until modern times that the very idea of #)
was imaginable or plausible. After Goedel's completeness and
incompleteness theorems, there was a general consensus that #) exists
and is rather substantive. Later events strengthened this consensus.

Once one adopts #), scientific objects such as PA = Peano Arithmetic
are inevitable from many directions.

I offer up some facts about PA of a certain kind that show PA "in
action".

THEOREM 1. Let n >> k. For any x_1,x_2,...,x_n from {1,...,k}, there
exists 1 <= i < j <= n/2 such that the block x_i,...,x_2i is a
subsequence (allowing skips) of the block x_j,...,x_2j.

THEOREM 2. Let n >> k. For any f:{1,...,n}^k into {1,...,n} obeying
f(x) <= max(x), there exists 1 <= x_1 < ... < x_k+1 <= n such that
f(x_1,...,x_k) <= f(x_2,...,x_k+1).

THEOREM 3. Let n >> k. For any f:{1,...,n}^k into {1,...,n} obeying
f(x) <= max(x), there exists x_1 < ... < x_k+2 such that
f(x_1,...,x_k) <= f(x_2,...,x_k+1) <= f(x_3,...,x_k+1).

THEOREM 4. Theorem 1 is provable in PA(3) but not in PA(2). I.e., in
PA with the 3 quantifier induction scheme, but not in PA with the 2
quantifier induction scheme.

THEOREM 5. Theorem 2 is provable in PA, and in fact, in EFA =
exponential function arithmetic. Theorem 3 is not provable in PA, but
is provable in ACA', or even PA + 1-Con(PA).

Harvey Friedman

```