

A056041


Value for which b(a(n))=0 when b(2)=n and b(k+1) is calculated by writing b(k) in base k, reading this as being written in base k+1 and then subtracting 1.


8




OFFSET

0,1


COMMENTS

a(8)=3*2^(3*2^27+27)1 which is more than 10^(10^8) and equal to the final base of the Goodstein sequence starting with g(2)=4; indeed, apart from the initial term, the sequence starting with b(2)=8 is identical to the Goodstein sequence starting with g(2)=4. The initial terms of a(n) [2, 3, 5 and 7] are equal to the initial terms of the equivalent final bases of Goodstein sequences starting at the same points. a(9)=2^(2^(2^70+70)+2^70+70)1 which is more than 10^(10^(10^20)).
It appears that if n is even then a(n) is one less than three times a power of two, while if n is odd then a(n) is one less than a power of two.
Comment from John Tromp, Dec 02 2004: The sequence 2,3,5,7,3*2^402653211  1, ... gives the final base of the Goodstein sequence starting with n. This is an example of a very rapidly growing function that is total (i.e. defined on any input), although this fact is not provable in firstorder Peano Arithmetic. See the links for definitions. This grows even faster than the Friedman sequence described in the Comments to A014221.
In fact there are two related sequences: (i) The Goodstein function l(n) = number of steps for the Goodstein sequence to reach 0 when started with initial term n >= 0: 0, 1, 3, 5, 3*2^402653211  3, ...; and (ii) the same sequence + 2: 2, 3, 5, 7, 3*2^402653211  1, ..., which is the final base reached. Both grow too rapidly to have their own entries in the database.
Related to the hereditary base sequences  see crossreference lines.


LINKS

Table of n, a(n) for n=0..7.
R. L. Goodstein, On the Restricted Ordinal Theorem, J. Symb. Logic 9, 3341, 1944.
L. Kirby, and J. Paris, Accessible independence results for Peano arithmetic, Bull. London Mathematical Society, 14 (1982), 285293.
J. Tromp, Programming Pearls
Eric Weisstein's World of Mathematics, Goodstein Sequence
Wikipedia, Goodstein's_theorem


EXAMPLE

a(3)=7 because starting with b(2)=3=11 base 2, we get b(3)=111 base 3=10 base 3=3, b(4)=101 base 4=3, b(5)=31 base 5=2, b(6)=21 base 6=1 and b(7)=11 base 7=0.


PROG

Concerning the sequence 2, 3, 5, 7, 3*2^402653211  1, ... mentioned above, John Tromp write: In Haskell, the sequence is the infinite list
main=mapM_(print.g 2)[0..] where
g b 0=b; g b n=g c(s 0 n1)where s _ 0=0; s e n=mod n b*c^s 0 e+s(e+1)(div n b); c=b+1
In Ruby, f(n) is defined by
def s(b, e, n)n==0?0:n%b*(b+1)**s(b, 0, e)+s(b, e+1, n/b)end
def g(b, n)n==0?b:g(b+1, s(b, 0, n)1)end
def f(n)g(2, n)end


CROSSREFS

Cf. Goodstein sequences: A056041 A056004 A059934 A057650 A056193 A059933 A059935 A059936; Woodall numbers: A003261.
Sequence in context: A120805 A177119 A096265 * A083017 A006510 A006055
Adjacent sequences: A056038 A056039 A056040 * A056042 A056043 A056044


KEYWORD

base,nonn


AUTHOR

Henry Bottomley, Aug 04 2000


STATUS

approved



