Gimel function

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In axiomatic set theory, the gimel function is the following function mapping cardinal numbers to cardinal numbers:

\gimel\colon\kappa\mapsto\kappa^{\mathrm{cf}(\kappa)}

where cf denotes the cofinality function; the gimel function is used for studying the continuum function and the cardinal exponentiation function. The symbol \gimel is a serif form of the Hebrew letter gimel.

The gimel hypothesis states that \gimel(\kappa)=\max(2^{\text{cf}(\kappa)},\kappa^+)

Values of the Gimel function

The gimel function has the property \gimel(\kappa)>\kappa for all infinite cardinals κ by König's theorem.

For regular cardinals \kappa, \gimel(\kappa)= 2^\kappa, and Easton's theorem says we don't know much about the values of this function. For singular \kappa, upper bounds for \gimel(\kappa) can be found from Shelah's PCF theory.

Reducing the exponentiation function to the gimel function

Bukovský (1965) showed that all cardinal exponentiation is determined (recursively) by the gimel function as follows.

  • If κ is an infinite regular cardinal (in particular any infinite successor) then 2^\kappa = \gimel(\kappa)
  • If κ is infinite and singular and the continuum function is eventually constant below κ then 2^\kappa=2^{<\kappa}
  • If κ is a limit and the continuum function is not eventually constant below κ then 2^\kappa=\gimel(2^{<\kappa})

The remaining rules hold whenever κ and λ are both infinite:

  • If ℵ0≤κ≤λ then κλ = 2λ
  • If μλ≥κ for some μ<κ then κλ = μλ
  • If κ> λ and μλ<κ for all μ<κ and cf(κ)≤λ then κλ = κcf(κ)
  • If κ> λ and μλ<κ for all μ<κ and cf(κ)>λ then κλ = κ

References

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  • Thomas Jech, Set Theory, 3rd millennium ed., 2003, Springer Monographs in Mathematics, Springer, ISBN 3-540-44085-2.