Kruskal's tree theorem

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. The theorem was conjectured by Andrew Vázsonyi and proved by Joseph Kruskal (1960); a short proof was given by Nash-Williams (1963). It has since become a prominent example in reverse mathematics as a statement that cannot be proved within ATR₀ (a form of arithmetical transfinite recursion), and a finitary application of the theorem gives the existence of the notoriously fast-growing TREE function.

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.

Statement

We give the version proved by Nash-Williams; Kruskal's formulation is somewhat stronger. Given a tree $T$ with a root, and given vertices $v$, $w$, call $v$ a successor of $w$ if the unique path from the root to $v$ contains $w$, and call $v$ an immediate successor of $w$ if additionally the path from $w$ to $v$ contains no other vertex. Take $X$ to be a partially ordered set. Taking $T_{1},T_{2}$ to be rooted trees with vertices labeled in $X$, we say that $T_{1}$ is inf-embeddable in $T_{2}$ and write $T_{1}\leq T_{2}$ if there is a map $F$ from the vertices of $T_{1}$ to the vertices of $T_{2}$ such that

• For all vertices $v$ of $T_{1}$, the label of $v$ precedes the label of $F(v)$,
• If $w$ is any successor of $v$ in $T_{1}$, then $F(w)$ is a successor of $F(v)$, and
• If $w_{1},w_{2}$ are any pair of distinct immediate successors of $v$, then the path from $F(w_{1})$ to $F(w_{2})$ in $T_{2}$ contains $F(v)$.

Then result states that, if $X$ is well-quasi-ordered, it follows that the set of rooted trees with labels in $X$ is well-quasi-ordered under the order defined above. That is to say, given any infinite sequence $T_{1},T_{2},\dots$ of rooted trees labeled in $X$, there is some $i<j$ so that $T_{i}\leq T_{j}$.

Weak tree function

Define tree(n), the weak tree function, as the length of the longest sequence of 1-labelled trees such that:

• Every tree at position k (for all k) has no more than k + n vertices.
• No tree is homeomorphically embeddable into any tree following it in the sequence.

It is known that tree(1) = 2, tree(2) = 5, and tree(3) > 262140, but TREE(3) (see below) is larger than tree^{treetreetreetree8(7)(7)(7)(7)}(7).

Friedman's work

For a countable label set $X$, 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 1980's, an early success of the then-nascent field of reverse mathematics. In the case where the trees above are taken to be unlabeled (that is, in the case where $X$ has order one), Friedman found that the result was unprovable in ATR₀, thus giving the first example of a predicative result with a provably impredicative proof. This case of the theorem is still provable in Π¹
₁-CA₀, but by adding a "gap condition" to the definition of the order on trees above, he found a natural variation of the theorem unprovable in this system. Much later, the Robertson-Seymour theorem would give another theorem unprovable inside Π¹
₁-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).

TREE(3)

Suppose that P(n) is the statement:

There is some m such that if T₁,...,T_m is a finite sequence of unlabeled rooted trees where T_k has n+k vertices, then T_i ≤ T_j for some i < j.

This statement is a simple application of Kruskal's theorem. For each n, Peano arithmetic can prove that P(n) is true, but Peano arithmetic cannot prove the statement "P(n) is true for all n". Moreover the shortest proof of P(n) in Peano arithmetic grows phenomenally fast as a function of n; far faster than any primitive recursive function or the Ackermann function for example. The least m for which P(n) holds similarly grows extremely quickly with n.

By incorporating labels, Friedman defined a far-faster growing function. For a positive integer n, take TREE(n) to be the largest m so that we have the following:

There is a sequence T₁,...,T_m of rooted trees labelled from a set of n labels, where each T_i has at most i vertices, such that T_i  ≤  T_j does not hold for any i < j  ≤ m.

The TREE sequence begins TREE(1) = 1, TREE(2) = 3, then suddenly TREE(3) explodes to a value so enormously large that many other "large" combinatorial constants, such as Friedman's n(4), are extremely small by comparison. In fact, it is much larger than nⁿ⁽⁵⁾(5). A lower bound for n(4), and hence an extremely weak lower bound for TREE(3), is A(A(...A(1)...)), where the number of A's is A(187196), and A() is a version of Ackermann's function: A(x) = 2 [x+1] x in hyperoperation. Graham's number, for example, is approximately A⁶⁴(4), which is much smaller than the lower bound A^A(187196)(1). It can be shown that the growth-rate of the function TREE is at least $f_{\theta (\Omega ^{\omega }\omega )}$ in the fast-growing hierarchy.

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 and n(3) > 2 [7199] 158386.

References

Citations

Bibliography

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• Kruskal, J. B. (May 1960), "Well-quasi-ordering, the tree theorem, and Vazsonyi's conjecture" (PDF), Transactions of the American Mathematical Society, American Mathematical Society, 95 (2): 210–225, doi:10.2307/1993287, JSTOR 1993287, MR 0111704
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