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Reference.org
Buchholz psi functions
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<p>Buchholz's psi-functions are a hierarchy of single-argument <a href="/facts/Ordinal_notation/7gjtASPb">ordinal functions</a> ψ ν ( α ) {\displaystyle \psi _{\nu }(\alpha )} introduced by German mathematician Wilfried Buchholz in 1986. These functions are a simplified version of the θ {\displaystyle \theta } -functions, but nevertheless have the same strength as those. Later on this approach was extended by Jäger and <a href="/facts/Kurt_Sch%25C3%25BCtte/I69va5Lm">Schütte</a>. </p>