Icosation refers to the 20th hyperoperation starting from addition. It is equal to \(a \uparrow^{18} b\) in Knuth's up-arrow notation.

Icosation can be written in array notation as \(\{a,b,18\}\) and in chained arrow notation as \(a \rightarrow b \rightarrow 18\).

Icosational growth rate is equivalent to \(f_19(n)\) in the fast-growing hierarchy.

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