Block subsequence theorem

The block subsequence theorem is a theorem due to Harvey Friedman unprovable in first order arithmetic. It shows that a string with a certain property involving subsequences cannot be infinite. The length of the largest string possible is a function of the alphabet size &mdash; the numbers that it generates immediately grow very large. One such number, dubbed n(4), is the subject of extensive research.

Suppose we have a string x1, x2, ... made of an alphabet of k letters, such that no block of letters xi, ..., x2i is a substring of any later block xj, ..., x2j. (We call such a string a Friedman string.) Friedman showed that such a string cannot be infinite, and for every k there is a sequence of maximal length. Call this length n(k).


 * \(n(1) = 3\), using the string "111". "1111" does not satisfy the conditions since x1, x2 = "11" is a subsequence of x2, x3, x4 = "111".
 * \(n(2) = 11\), using the string "12221111111".
 * \(n(3)\) and \(n(4)\) are much larger. Let A be a version of the Ackermann function as follows:
 * \(A(1, n) = 2n\)
 * \(A(k + 1, n) = 2 ↑^k n\)
 * \(A(n) = A(n, n)\)
 * \(A(7198, 158386) \leq n(3) \leq A(A(5))\)
 * \(n(4) \geq A^{A(187196)}(1)\)

Friedman has demonstrated that the growth rate of the n function lies asymptotically between ωω and ωω + 1 in the fast-growing hierarchy. Chris Bird notes that n(k) is "broadly comparable" to {3, 3,... 3} (with k 3's) in Bowers' array notation.

n(k) is related to the TREE sequence, which grows even faster.