The **block subsequence theorem** is a theorem due to Harvey Friedman.^{[1]}^{[2]}^{[3]} 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 — the numbers that it generates immediately grow very large. Two numbers in the sequence, **n(3)** and **n(4)**, are the subject of extensive research.

Suppose we have a string x_{1}, x_{2}, ... (We call such a string a *Friedman string*.) made of an alphabet of *k* letters, such that no block of letters x_{i}, ..., x_{2i} is a substring of any later block x_{j}, ..., x_{2j. }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*).

## Сomputation

n(k) is computable, and hence there must exists straightforward algorithm to calculate it:

- Start with 1.

- Rewrite number with k-ary positional system and keep it.
- Increase number by 1.
- Check whether this number represents Friedman string.
- If there is no number with m digits which represents Friedman string, then n(k)=m-1.

## Values

- \(n(1) = 3\), using the string "111". "1,111" does not satisfy the conditions since x
_{1}, x_{2}= "11" is a subsequence of x_{2}, x_{3}, x_{4}= "111". - \(n(2) = 11\), using the string "12,221,111,111" or "12,221,111,112".
- \(n(3)\) and \(n(4)\) are much larger. The strings which they used aren't known, but Friedman gave the string "12
^{2}131^{7}3^{2}1313^{8}13^{5}13^{20}1^{2}3^{53}13^{108}" = "122,131,111,111,331,313,333,333,313,333,313,333,333,333,333,333,333,311..."^{[4]}with length 216 for \(n(3)\).

- Define a version of the Ackermann function as follows:
- \(A(1, n) = 2n\)
- \(A(k + 1, n) = 2 \uparrow^k n\)
- \(A(n) = A(n, n)\)

- \(A(7\,198, 158\,386) < n(3) < A(A(5))\)
- \(n(4) > A^{A(187,196)}(1)\)

Friedman has demonstrated that the growth rate of the *n* function lies asymptotically between \(f_{ω^ω}(n)\) and \(f_{ω^ω+1}(n)\) in the fast-growing hierarchy. He has also stated that every n(k) is odd.^{[5]} Chris Bird notes that *n(k)* is "broadly comparable" to {3, 3, ..., 3, 3} (with k 3's) in Bowers' array notation. By contrast, Graham's number is only about {3, 64, 1, 2}.

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

## Sources

- ↑ Harvey Friedman, Long Finite Sequences
- ↑ Harvey Friedman, Enormous Integers in Real Life
- ↑ Large Numbers (page 4) at MROB
- ↑ A lower bound for n(3)
- ↑ http://www.personal.psu.edu/t20/fom/postings/9810/msg00094.html

## See also

**By Harvey Friedman:** Mythical tree problem · Friedman's vector reduction problem · Friedman's finite ordered tree problem · **block subsequence theorem** n(4) · Friedman's circle theorem · TREE sequence TREE(3) · subcubic graph number SCG(13) · transcendental integer · finite promise games · Friedman's finite trees · Greedy clique sequence**Miscellaneous:** Factorial · Folkman's number · Exploding Tree Function · Graham's number · fusible number