## FANDOM

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U(n) is an extremely fast-growing function devised by Chris Bird.[1] It is the fastest function he defined as of June 2013. Its growth rate is around $$\theta(\Omega_\omega)$$ in the fast-growing hierarchy.

Using Bird's Hierarchical Hyper-Nested Array Notation, $$U(n) = \{3,n [1 [1 [1 [ \cdots [1 [1 /_n 1 /_n 2] 2] \cdots ] 2] 2] 2] 2\}$$ (with n sets of square brackets).