Enneation or nonation refers to the 9th hyperoperation starting from addition. It is equal to the binary function \(\uparrow^{7}\) in Knuth's up-arrow notation.[1]

Enneation can be written in array notation as \(\{a,b,7\}\), in chained arrow notation as \(a \rightarrow b \rightarrow 7\) and in Hyper-E notation as E[a]1#1#1#1#1#1#b.

Enneational growth rate is equivalent to \(f_8(n)\) in the fast-growing hierarchy.

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