On a universal Σ-function over a tree
A. N. Khisamiev · Siberian Mathematical Journal · 2012
We obtain a sufficient condition for the absence of any universal Σ-function in an admissible set (a hereditarily finite admissible set). We construct a tree T of height 4 such that no universal Σ-function exists in the hereditarily finite admissible set ℍ $$\mathbb{F} $$ (T) over T.