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Terrible tethrathoth is equal to E100(#^^#)^^#100 = E100(#^^#)^(#^^#)...(#^^#)^(#^^#)100 (with 100 #^^#'s) in Extended Cascading-E Notation.[1][2][3] The term was coined by Sbiis Saibian.

## Etymology

The name of this number is based on the prefix "terrible" and the number "tethrathoth".

## Approximations in other notations

Notation Approximation
BEAF $$\{100,100 (X \uparrow\uparrow (2X)) 2\}$$[4]
Bird's array notation $$\{100,100 [1 \backslash 3] 2\}$$
X-Sequence Hyper-Exponential Notation $$100\{(X\uparrow\uparrow X)\uparrow\uparrow X\}100$$
Fast-growing hierarchy $$f_{\varepsilon_1}(100)$$
Hardy hierarchy $$H_{\varepsilon_1}(101)$$
Slow-growing hierarchy $$g_{\vartheta(\varepsilon_{\Omega 2 + 1})}(100)$$

## Sources

1. Saibian, Sbiis. 4.3.5 Cascading-E NumbersOne to Infinity. Retrieved 2016-09-09.
2. Sbiis Saibian's Ultimate Large Numbers List - Part III
3. Saibian, Sbiis. 4.3.7 xec_numbersOne to Infinity. Retrieved 2017-01-02.
4. Using particular notation $$\{a,b (A) 2\} = A\ \&\ a$$ with prime b.