Kruskal's tree theorem explained

In mathematics, Kruskal's tree theorem states that the set of finite trees over a well-quasi-ordered set of labels is itself well-quasi-ordered under homeomorphic embedding.

History

The theorem was conjectured by Andrew Vázsonyi and proved by ; a short proof was given by . It has since become a prominent example in reverse mathematics as a statement that cannot be proved in ATR₀ (a second-order arithmetic theory with a form of arithmetical transfinite recursion).

In 2004, the result was generalized from trees to graphs as the Robertson–Seymour theorem, a result that has also proved important in reverse mathematics and leads to the even-faster-growing SSCG function, which dwarfs

TREE ( 3 ) . A finitary application of the theorem gives the existence of the fast-growing TREE function.

Statement

The version given here is that proven by Nash-Williams; Kruskal's formulation is somewhat stronger. All trees we consider are finite.

Given a tree with a root, and given vertices,, call a successor of if the unique path from the root to contains, and call an immediate successor of if additionally the path from to contains no other vertex.

Take to be a partially ordered set. If, are rooted trees with vertices labeled in, we say that is inf-embeddable in and write

T _{1} \leq T _{2} if there is an injective map from the vertices of to the vertices of such that:

• For all vertices of, the label of precedes the label of

F(v) ;

• If is any successor of in, then

F(w) is a successor of F(v) ; and

• If, are any two distinct immediate successors of, then the path from

F(w _{1)} to F(w _{2)} in contains F(v) .

Kruskal's tree theorem then states:

If is well-quasi-ordered, then the set of rooted trees with labels in is well-quasi-ordered under the inf-embeddable order defined above. (That is to say, given any infinite sequence of rooted trees labeled in, there is some

i<j so that T _{i} \leq T _{j} .)

Friedman's work

For a countable label set, Kruskal's tree theorem can be expressed and proven using second-order arithmetic. However, like Goodstein's theorem or the Paris–Harrington theorem, some special cases and variants of the theorem can be expressed in subsystems of second-order arithmetic much weaker than the subsystems where they can be proved. This was first observed by Harvey Friedman in the early 1980s, an early success of the field of reverse mathematics. In the case where the trees above are taken to be unlabeled (that is, in the case where has size one), Friedman found that the result was unprovable in ATR₀,^[1] thus giving the first example of a predicative result with a provably impredicative proof.^[2] This case of the theorem is still provable by Π-CA₀, but by adding a "gap condition"^[3] to the definition of the order on trees above, he found a natural variation of the theorem unprovable in this system.^{[4] [5]} Much later, the Robertson–Seymour theorem would give another theorem unprovable by Π-CA₀.

Ordinal analysis confirms the strength of Kruskal's theorem, with the proof-theoretic ordinal of the theorem equaling the small Veblen ordinal (sometimes confused with the smaller Ackermann ordinal).

Weak tree function

Suppose that

P(n) is the statement:

There is some such that if is a finite sequence of unlabeled rooted trees where has

i + n vertices, then T _{i} \leq T _{j} for some i<j .

All the statements

P(n) are true as a consequence of Kruskal's theorem and Kőnig's lemma. For each, Peano arithmetic can prove that P(n) is true, but Peano arithmetic cannot prove the statement "P(n) is true for all ".^[6] Moreover, the length of the shortest proof of P(n) in Peano arithmetic grows phenomenally fast as a function of, far faster than any primitive recursive function or the Ackermann function, for example. The least for which P(n) holds similarly grows extremely quickly with .

Define

tree (n) , the weak tree function, as the largest so that we have the following:

There is a sequence of unlabeled rooted trees, where each has at most

i + n vertices, such that T _{i} \leq T _{j} does not hold for any i<j\leq m .

It is known that

tree ( 1 ) =2 , tree ( 2 ) =5 , tree ( 3 ) \geq 844 , 424 , 930 , 131 , 960 (about 844 trillion), tree ( 4 ) \gg g _{64} (where g _{64} is Graham's number), and TREE ( 3 ) (where the argument specifies the number of labels; see below) is larger than tree ^{tree ^{tree ^{tree ^{tree ^{8} ( 7 )} ( 7 )} ( 7 )} ( 7 )} ( 7 ).

To differentiate the two functions, "TREE" (with all caps) is the big TREE function, and "tree" (with all letters in lowercase) is the weak tree function.

TREE function

By incorporating labels, Friedman defined a far faster-growing function. For a positive integer , take

TREE (n) to be the largest so that we have the following:

There is a sequence of rooted trees labelled from a set of labels, where each has at most vertices, such that

T _{i} \leq T _{j} does not hold for any i<j\leq m .

The TREE sequence begins

TREE ( 1 ) =1 , TREE ( 2 ) =3 , then suddenly, TREE ( 3 ) explodes to a value that is so big that many other "large" combinatorial constants, such as Friedman's n( 4 ) , n ^{n(5)} ( 5 ) , and Graham's number, are extremely small by comparison. A lower bound for n( 4 ) , and, hence, an extremely weak lower bound for TREE ( 3 ) , is A ^{A(187196)} ( 1 ) . Graham's number, for example, is much smaller than the lower bound A ^{A(187196)} ( 1 ) , which is approximately g _{3 \uparrow ^{187196} 3} , where g _{x} is Graham's function.

See also

• Paris–Harrington theorem
• Kanamori–McAloon theorem
• Robertson–Seymour theorem

Notes

Friedman originally denoted this function by TR[''n''].

n(k) is defined as the length of the longest possible sequence that can be constructed with a k-letter alphabet such that no block of letters x_i,...,x_2i is a subsequence of any later block x_j,...,x_2j.

n( 1 ) = 3 , n( 2 ) = 11 , rm{and} n( 3 ) > 2 \uparrow ^{7197} 158386 .

A(x) taking one argument, is defined as A(x, x), where A(k, n), taking two arguments, is a particular version of Ackermann's function defined as: A(1, n) = 2n, A(k+1, 1) = A(k, 1), A(k+1, n+1) = A(k, A(k+1, n)).

References

CitationsBibliography

Notes and References

1. Simpson 1985, Theorem 1.8
2. Friedman 2002, p. 60
3. Simpson 1985, Definition 4.1
4. Simpson 1985, Theorem 5.14
5. Marcone 2001, p. 8–9
6. Smith 1985, p. 120