The expofactorial function is like the factorial function but with exponents.
n ! = n ( n − 1 ) !
For example 4 ! = 4 3 2 1 = 4 9 = 2 6 2 1 4 4
Using Knuth's up arrow notation
b ↑ 1 d = b d , b ↑ n 0 = 1 and b ↑ n d = b ↑ n − 1 [ b ↑ 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 = 6 5 5 3 6
Give the minimum integer of n such that
3 ↑ n 3 > 2 0 0 !
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2 0 0 ! is pretty large, but it is under 1 0 ↑ ↑ 1 9 9
But for n = 3 we have 3 ↑ 3 3 = 3 ↑ ↑ ↑ 3 = 3 ↑ ↑ [ 3 ↑ ↑ ↑ 2 ]
= 3 ↑ ↑ [ 3 ↑ ↑ 3 ]
= 3 ↑ ↑ [ 3 ↑ 3 ↑ 3 ]
= 3 ↑ ↑ [ 3 2 7 ]
= 3 ↑ 3 ↑ 3 . . . ↑ 3 ↑ 3 with 7625597484987 3 's. It shouldn't be hard to see that this is much, much larger.
In fact, it is greater than 1 0 ↑ ↑ ( 1 0 ↑ 1 0 )