Question about arithmetical hierarchy notation

Hello :)

I am awfully curious if anyone knows why we use the symbols we do
for arithmetical hierarchical classes. I mean of course Sigma_n and
Pi_n. Is there a particular reason for these symbols or is it just
arbitrary? If the former is true, any recommended reading?

I love computability theory, it is totally sweet. I can't get my
hands on enough computability theory literature, this university needs
to build a special library devoted to computability theory!

Snis Pilbor

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