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N(2) tree

Notes: Tree shows only acceptable sequences inscribed in each node;
Inverse sequences (where A's replaced by B's and vice versa) aren't shown;
Painted purple nodes are members of the longest sequence.

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 x1, x2, ... (We call such a string a Friedman string.) made of an alphabet of k letters, such that no block of letters xi, ..., x2i is a substring of any later block xj, ..., x2j. 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 Edit

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

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

Values Edit

  • \(n(1) = 3\), using the string "111". "1,111" does not satisfy the conditions since x1, x2 = "11" is a subsequence of x2, x3, x4 = "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 "1221317321313813513201235313108" = "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:
    \(A(1, n) = 2n\)
    \(A(k + 1, n) = 2 \uparrow^k n\)
    \(A(n) = A(n, n)\)
    • \(A(7198, 158386) < n(3) < A(A(5))\)
    • \(n(4) > A^{A(187,196)}(1)\)

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 Edit

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

See also Edit

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