Thresholds for Ackermannian Ramsey Numbers

1 / 47

# Thresholds for Ackermannian Ramsey Numbers - PowerPoint PPT Presentation

Thresholds for Ackermannian Ramsey Numbers. Authors: Menachem Kojman Gyesik Lee Eran Omri Andreas Weiermann. Notation …. n = {1..n} means: for every coloring C of the edges of the complete graph K n , there is a complete Q monochromatic sub-graph of size k .

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about ' Thresholds for Ackermannian Ramsey Numbers' - milo

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Thresholds for Ackermannian Ramsey Numbers

Authors: Menachem Kojman

Gyesik Lee

Eran Omri

Andreas Weiermann

Notation…
• n = {1..n}
• means:

for every coloring C of the edges of the complete graph Kn, there is a complete Q monochromatic sub-graph of size k.

n–Size of complete graph over which we color edges.

k–Size of homogeneous sub-graph.

c–Number of colors.

2–Size of tuples we color – pairs (edges).

2

Q is homogeneous for C

Example

Example…

Let us prove:

• means:

for every coloring C of the edges of the complete graph Kn, there is a complete Q monochromatic sub-graph of size k.

n = 22k-1–Size of complete graph over which we color edges.

k–Size of homogeneous sub-graph.

c = 2–Number of colors.

Proof…
• Let us, from now on, assume the vertex set of every graph we consider is some initial segment of the natural numbers.
• First step:

Find amin-homogeneouscomplete sub-graph of size2k.

Definition:A complete graph G = (E,V), with the natural ordering on V, is min-homogeneous for a coloring

if for every v in V, all edges (v,u), for

u > v, are assigned one color.

…Proof…

Let G = (V,E) be the complete graph with V= 22k-1:

x2

x3

x4

x5

x6

x1

x22k-1

And let C be a coloring of the vertices of G with 2colors (Red, Blue).

C(x1,x2) = redC(x2,x6) = blue

…Proof…

Markxiwith the colorC(x1, xi)

x2

x3

x4

x5

x1

x6

x22k-1

There is a monochromatic complete sub-graph of{x2,x3,…} of size 22k-2. (Sayred)

…Proof…

Mark xi with the colorC(x2, xi)

x2

x3

x6

x1

There is a monochromatic complete sub-graph of{x3,x6,…} of size 22k-3. (Sayblue)

…Proof…

Now, we have amin-homogeneoussequence {xi1, xi2, xi3, …, xi2k}

xi2

xi3

xi4

xi5

xi1

xi6

xi2k

Mark xiawith C(xia, xib) for allb > a.

There is a monochromatic subset of

{xi1, xi2, xi3, … xi2k} of size k.

• Second (and final) step:
• Find a k-sizedhomogeneouscomplete sub-graph
Ramsey Numbers.
• We Showed:
• On the other Hand: -Using the Probabilistic Method, we can show:

Denote Rc(k) :=The minimumnto satisfy:

To sum up:

R2(k) – Exponential in k

More importantly:
• Explanation: - For a k-sized sequence iterate step #1 (repeated division) k times (namely, divide by at most c at each iteration)
Primitive Recursive Functions and Ackermann’s Function

A function that can be implemented using only for-loops is calledprimitive recursive.

Ackermann’s function– A simple example of a well defined total function that is computable but not primitive recursive

Regressive Ramsey….
• g-regressive colorings:

A coloring C is g-regressive if for every (m,n)

C(m,n) ≤ g(min(m,n)) = g(m)

• Can we still demand homogeneity??
• Not necessarily!!! ( e.g C(m,n) = Id(m) )

Observe that

Is true for any g: N  N which can be established by means,similar to the regular Ramsey proof and compactness.

The g-regressive Ramsey Number

Denote Rg(k) :=The minimumnto satisfy:

We have seen so far:

For a constant function,g(x) = c

Rg(k) ≤ ck Since

On the other hand it was known…

That for g = ID:

Rg(k) is Ackermannian in terms of k.

Namely, DOMINATES every primitive recursive function.

The Problem
• constant g

Rg(k) < gk (primitive recursive.)

• Threshold g

Rg(k) is ackermannian.

• g = Id
The Results

If g(n) is ‘fast’ to go below n1/kthen, Rg(k) is primitive recursive

If g(n)≤n1/kthen,

x1/j - Ackermannian

I will insert a drawing

The Results

Suppose B : N  N is positive, unbounded and non-decreasing.

Let gB(n) = n1/B−1(n). Where

B-1(n) = min{t : B(t) ≥ n}. Then,

RgB(k) is Ackermannian

iff

B is Ackermannian.

Min-homogeneity – Lower Threshold

Basic Pointers:

• Assume more colors.

Set c = gB(n)

• Use repeated division to show

min

B(k) ≥ ck (k)gB

Suppose B : N  N is positive, unbounded and non-decreasing.

Let gB(n) = n1/B−1(n). Where

B-1(n) = min{t : B(t) ≥ n}. Then,

for every natural number k, it holds that

RgB(k)≤ B(k).

Min-homogeneity – Upper Threshold
• We show:
• To prove this, we present, given k,

a bad coloring of an Akermannianly large n

C(m,n) = <I,D>

I Largest i s.t m,n are not in same segment.

D  Distance between m’s segment and n’s.

(fg)i(µ)

k

(fg)3(2)(µ)

i

(fg)3(µ)

3

2

1

µµ+1 µ+2 …

{(fg)i}

C(m,n) = <I,D> = <2,1>

I Largest i s.t m,n are not in same segment.

D  Distance between m’s segment and n’s.

k

i

3

2

1

µµ+1 µ+2 …

m = µ+8

n = µ+29

The Coloring – Formal Definition

Given a monotonically increasing function 4g2and a natural number k >2 with

we define a coloring

That is:

• 4g2-regressive on the interval
• Has no min-homogeneous set of size k+1 within that interval.
The Coloring…
• So, why is it:
• 4g2-regressive?
• Avoiding a min-homogeneous set?
The Results - Surfing the waves

x1/j - Ackermannian

I will insert a drawing

In 1985 Kanamori & McAloon

Had used means of Model Theory to show that the bound of :

eventually DOMINATES every primitive recursive function.

In 1991 Prömel, Thumser & Voigt and independently in 1999 Kojman & Shelah have presented two simple combinatorial proofs to this fact.

Primitive Recursive Functions

Input:Tuples of natural numbers

Output:A natural number

Basic primitive recursive functions:

• The constant function 0
• The successor function S
• The projection functions Pin(x1, x2,…, xn) = xi
Primitive Recursive Functions

More complex primitive recursive functions are obtained by :

• Composition:h(x0,...,xl-1) = f(g0(x0,...,xl-1),...,gk-1(x0,...,xl-1))
• Primitive recursion:h(0,x0,...,xk-1) = f(x0,...,xk-1) h(S(n),x0,...,xk-1) = g(h(n,x0,...,xk-1),n,x0,...,xk-1)
Given a function g : N N, denote

General Definition – (fg) Hierarchy

Where f0(n) = n and fj+1(n) = f(fj(n))

Let g = Id. Now

Ackermann’s Function

Denote: Ack(n) = An(n)

Examples:

Ackermann’s Function

Infinite Canonical Ramsey theorem

Definition…

Examples…

• Finite Canonical Ramsey theorem

1. clearly:

2. On the other handthere exist t,l such that:

Now, since

We have

And thus:

k

i

3

2

1

µµ+1 µ+2 …

There exists no:

Which is min-homogeneous for Cg... But, for every (m,n) in the interval,

Homogeneity – Lower Bound
• We show:
• To prove that we used:
Homogeneity – Upper Bound
• We showed:

To do that we used a general, well known, coloring method…

Example…
• The rules:
• Any two players may choose to play Pool or Snooker. (Coloring pairs with two colors)
• A tournament can take place either in Snooker or in Pool. All the couples must choose the same. (Homogeneous set)
• Three players minimum. (Size of subset)

Imagine yourself in a billiard hall…

A tournament is being organized…

How many players will ensure a Tournament??

6

Ramsey Number for 3 is 6.

Pool =

Snooker =

Given a function g : N N, denote

General Definition – (fg) Hierarchy

Where f0(n) = n and fj+1(n) = f(fj(n))

Suppose g : N  N is nondecreasing and unbounded. Then,

Rg(k) is bounded by some primitive recursive function in k

iff

for every t > 0 there is some M(t) s.t for all n ≥ M(t) it holds that g(n) < n1/t and M(t) is primitive recursive in t.