Two very large numbers

The expofactorial function is like the factorial function but with exponents.

n ! = n ( n 1 ) ! \large n^{!}=n^{(n-1)^{!}}

For example 4 ! = 4 3 2 1 = 4 9 = 262144 4^{!}=4^{3^{2^{1}}}=4^{9}=262144

Using Knuth's up arrow notation

b 1 d = b d b \uparrow^{1} d = b^{d} , b n 0 = 1 b \uparrow^{n} 0 =1 and b n d = b n 1 [ b n ( d 1 ) ] b \uparrow^{n} d = b \uparrow^{n-1} [b \uparrow^{n} (d-1)]

For example 2 2 4 = 2 1 [ 2 2 3 ] = 2 [ 2 [ 2 2 2 ] ] = 2 [ 2 [ 2 [ 2 2 1 ] ] ] = 2 2 2 2 = 65536 2 \uparrow^{2} 4 = 2 \uparrow^{1} [2 \uparrow^{2} 3] = 2 \uparrow [2 \uparrow [2 \uparrow^{2} 2]]=2 \uparrow [2 \uparrow [2 \uparrow [2 \uparrow^{2} 1]]] = 2^{2^{2^{2}}} = 65536

Give the minimum integer of n n such that

3 n 3 > 20 0 ! \huge 3 \uparrow^{n} 3 > 200^{!}

100 101 4 200! 67 3 200

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1 solution

Jeremy Galvagni
Aug 24, 2018

20 0 ! 200^{!} is pretty large, but it is under 10 199 10 \uparrow \uparrow 199

But for n = 3 n=3 we have 3 3 3 = 3 3 = 3 [ 3 2 ] 3 \uparrow^{3} 3 = 3 \uparrow \uparrow \uparrow 3 = 3 \uparrow \uparrow [3 \uparrow \uparrow \uparrow 2]

= 3 [ 3 3 ] =3 \uparrow \uparrow [3 \uparrow \uparrow 3]

= 3 [ 3 3 3 ] =3 \uparrow \uparrow [3 \uparrow 3 \uparrow 3]

= 3 [ 3 27 ] =3 \uparrow \uparrow [3^{27}]

= 3 3 3... 3 3 = 3 \uparrow 3 \uparrow 3 ... \uparrow 3 \uparrow 3 with 7625597484987 3 3 's. It shouldn't be hard to see that this is much, much larger.

In fact, it is greater than 10 ( 10 10 ) 10 \uparrow \uparrow (10 \uparrow 10)

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