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Pentacthulhum (also called tethrarxihect[1]) is equal to E100#^^^#100 = E100#^^#^^#...#^^#^^#100 (100 #'s) in Extended Cascading-E Notation.[1] The term was coined by Sbiis Saibian.

This expands to:

E100#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#^^#100

Etymology

The name of this number is based on the words "pentation" and "Cthulhu".

Approximations in other notations

Notation Approximation
BEAF $$\{X,X,1,2\}\ \&\ 100$$ (weaker bound)

$$\{X,X,3\}\ \&\ 100$$ (stronger bound)

Bird's array notation $$\{100,100[1[1\neg3]2]2\}$$
Hyperfactorial array notation $$100![1(1)2]$$
Dollar function $$100[[[[0]_2]_2]_2]$$
Fast-growing hierarchy (with this system of fundamental sequences) $$f_{\varphi(1,0,0)}(99)$$
$$=f$$$$_{\Gamma_0}$$$$(99)$$
Hardy hierarchy (with this system of fundamental sequences) $$H_{\varphi(1,0,0)}(100)$$
$$= H_{\Gamma_{0}}(100)$$
Slow-growing hierarchy $$g_{\vartheta(\Omega_2)}(100)$$

Sources

1. Saibian, Sbiis. 4.3.9 - Extended Cascading-E Numbers Part III (9010 - 11870). Retrieved 2018-02-22.