Undecation is the 11th hyperoperation, which is equal to \(a \uparrow^{9} b\) in Knuth's up-arrow notation. The n undecated to m is n decated to n decated to ... decated to n (m n's). Undecation can be written in BEAF as \(\{a,b,9\}\), in chained arrow notation as \(a ightarrow b ightarrow 9\) and in Hyper-E notation as E(a)1#1#1#1#1#1#1#1#b. Undecational growth rate is comparable to \(f_{10}(n)\) in the fast-growing hierarchy.