User:Sligocki/Green's numbers

Let us define the numbers ${\displaystyle B_{n}}$ for n odd.

• ${\displaystyle B_{n}(0)=1}$
• ${\displaystyle B_{1}(m)=m+1}$
• ${\displaystyle B_{n}(m)=B_{n-2}[B_{n}(m-1)+1]+1}$

Then, Green's numbers ${\displaystyle BB_{n}}$ are defined as:

• ${\displaystyle BB_{n}=B_{n-2}[B_{n-2}(1)]}$ for odd n
• ${\displaystyle BB_{n}=B_{n-3}[B_{n-3}(3)+1]+1}$ for even n

• ${\displaystyle B_{1}(m)=m+1}$
• ${\displaystyle B_{3}(m)=3m+1}$
• ${\displaystyle B_{5}(m)={\frac {7}{2}}\cdot 3^{m}-{\frac {5}{2}}}$
• ${\displaystyle B_{7}(m)>3\uparrow \uparrow m}$ and ${\displaystyle B_{7}(1)=B_{5}(2)+1=29}$
• ${\displaystyle B_{9}(m)>3\uparrow \uparrow \uparrow m}$ and ${\displaystyle B_{9}(1)=B_{7}(2)+1=B_{5}(30)+2=720618962331271}$

• ${\displaystyle BB_{3}=3}$
• ${\displaystyle BB_{4}=7}$
• ${\displaystyle BB_{5}=13}$
• ${\displaystyle BB_{6}=35}$
• ${\displaystyle BB_{7}=B_{5}(8)=22961}$
• ${\displaystyle BB_{8}=B_{5}(93)+1=3(7\cdot 3^{92}-1)/2=824792557184288824246737061810550733633916929}$
• ${\displaystyle BB_{9}=B_{7}(B_{7}(1))=B_{7}(29)>3\uparrow \uparrow 29>10\uparrow \uparrow 28}$ > a googolplex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex-plex (25 plexes).
• ${\displaystyle BB_{10}=B_{7}(B_{7}(3)+1)+1>3\uparrow \uparrow 3\uparrow \uparrow 3=3\uparrow \uparrow \uparrow 3}$ = the third Ackermann number
• ${\displaystyle BB_{11}=B_{9}(B_{9}(1))=B_{9}(720618962331271)>3\uparrow \uparrow \uparrow 720618962331271}$

We will show that ${\displaystyle B_{n}(m)}$ grows like ${\displaystyle 3\uparrow ^{n/2}m}$ and thus that ${\displaystyle BB_{n}}$ grows like ${\displaystyle 3\uparrow ^{n/2}3}$ (See Knuth's up-arrow notation and User:sligocki/up-arrow properties).

Claim.

${\displaystyle B_{2k+3}(m)\geq 3\uparrow ^{k}m}$ for any ${\displaystyle k\geq -2}$ and ${\displaystyle m\geq 0}$

Proof by induction.

Base Case

${\displaystyle B_{2k+3}(0)=1\geq 1=3\uparrow ^{k}0}$

${\displaystyle B_{1}(m)=m+1\geq m+1=3\uparrow ^{-2}m}$

Inductive Step

Assume that ${\displaystyle B_{2k^{\prime }+3}(m^{\prime })\geq 3\uparrow ^{k^{\prime }}m^{\prime }}$ (for all ${\displaystyle k^{\prime }=k,m^{\prime }<m}$ or ${\displaystyle k^{\prime }<k,m^{\prime }<3\uparrow ^{k}(m-1)}$ )

${\displaystyle B_{2k+3}(m)=B_{2k+1}[B_{2k+3}(m-1)+1]+1>B_{2k+1}[3\uparrow ^{k}(m-1)]>3\uparrow ^{k-1}[3\uparrow ^{k}(m-1)]=3\uparrow ^{k}m\,}$

QED

Claim.

${\displaystyle B_{2k+3}(m)<{\frac {1}{2}}(3\uparrow ^{k}(m+2))<(3\uparrow ^{k}(m+2))}$ for any ${\displaystyle k\geq 1}$ and ${\displaystyle m\geq 0}$ (In fact the bound can be tightened to m+1 for k ≥ 2)

Proof by induction.

Base Case

${\displaystyle B_{2k+3}(0)=1<{\frac {1}{2}}(3\uparrow ^{k}2)}$

${\displaystyle B_{5}(m)={\frac {9}{2}}3^{m}<{\frac {1}{2}}(3\uparrow (m+2))}$

${\displaystyle B_{7}(m)<{\frac {1}{2}}(3\uparrow ^{2}(m+1))}$ (left as an exercise to the reader)

Inductive Step

Assume that ${\displaystyle B_{2k^{\prime }+3}(m^{\prime })<{\frac {1}{2}}(3\uparrow ^{k^{\prime }}(m^{\prime }+2))}$ (for ${\displaystyle k^{\prime }=k,m^{\prime }<m}$ or ${\displaystyle k^{\prime }<k,m^{\prime }<3\uparrow ^{k}m+1}$ )

${\displaystyle {\begin{array}{rll}B_{2k+3}(m)=B_{2k+1}[B_{2k+3}(m-1)+1]+1&<&B_{2k+1}[{\frac {1}{2}}(3\uparrow ^{k}(m+1))+1]+1\\&<&{\frac {1}{2}}(3\uparrow ^{k-1}[{\frac {1}{2}}(3\uparrow ^{k}(m+1))+3])+1\\&\leq ?&{\frac {1}{2}}(3\uparrow ^{k-1}[{\frac {1}{2}}(3\uparrow ^{k}(m+1))+4])\\&\leq ?&{\frac {1}{2}}(3\uparrow ^{k-1}(3\uparrow ^{k}(m+1)))\\&=&{\frac {1}{2}}(3\uparrow ^{k}(m+2))\end{array}}}$

≤? statements seem obvious, but grungy to prove (left as an exercise to the reader :)

Claim.

${\displaystyle 3\uparrow ^{k}3<BB_{2k+4},BB_{2k+5}<3\uparrow ^{k+1}3}$ for ${\displaystyle k\geq 1}$

Proof.

${\displaystyle BB_{2k+5}=B_{2k+3}(B_{2k+3}(1))>3\uparrow ^{k}(3\uparrow ^{k}1)=3\uparrow ^{k}3}$

${\displaystyle BB_{2k+4}>B_{2k+1}(B_{2k+1}(3))>3\uparrow ^{k-1}(3\uparrow ^{k-1}3)=3\uparrow ^{k}3}$

${\displaystyle BB_{2k+5}=B_{2k+3}(B_{2k+3}(1))<{\frac {1}{2}}(3\uparrow ^{k}({\frac {1}{2}}(3\uparrow ^{k}(1+2))+2))<3\uparrow ^{k}(3\uparrow ^{k}3)=3\uparrow ^{k+1}3}$

${\displaystyle BB_{2k+4}=B_{2k+1}[B_{2k+1}(3)+1]+1<{\frac {1}{2}}(3\uparrow ^{k-1}({\frac {1}{2}}(3\uparrow ^{k-1}(3+2))+3))+1<3\uparrow ^{k-1}(3\uparrow ^{k-1}5)<3\uparrow ^{k}4<3\uparrow ^{k+1}3}$

QED