# Sh:513

- Shelah, S. (2002).
*PCF and infinite free subsets in an algebra*. Arch. Math. Logic,**41**(4), 321–359. arXiv: math/9807177 DOI: 10.1007/s001530100101 MR: 1906504 -
Abstract:

We give another proof that for every \lambda\geq\beth_\omega for every large enough regular \kappa<\beth_\omega we have \lambda^{[\kappa]}=\lambda, dealing with sufficient conditions for replacing \beth_\omega by \aleph_\omega. In §2 we show that large pcf(\mathfrak{a}) implies existence of free sets. An example is that if pp(\aleph_\omega)>\aleph_{\omega_1} then for every algebra M of cardinality \aleph_\omega with countably many functions, for some a_n\in M (for n<\omega) we have a_n\notin c\ell_M(\{a_l: l\neq n, l<\omega\}). Then we present results complementary to those of section 2 (but not close enough): if IND(\mu,\sigma) (in every algebra with universe \lambda and \le\sigma functions there is an infinite independent subset) then for no distinct regular \lambda_i\in {\rm Reg}\setminus\mu^+ (for i<\kappa) does \prod_{i<\kappa}\lambda_i/[\kappa]^{\le\sigma} have true cofinality. We look at IND(\langle J^{bd}_{\kappa_n}: n<\omega\rangle) and more general version, and from assumptions as in §2 get results even for the non stationary ideal. Lastly, we deal with some other measurements of [\lambda]^{\ge \theta} and give an application by a construction of a Boolean Algebra. - Version 2002-04-03_10 (56p) published version (39p)

Bib entry

@article{Sh:513, author = {Shelah, Saharon}, title = {{PCF and infinite free subsets in an algebra}}, journal = {Arch. Math. Logic}, fjournal = {Archive for Mathematical Logic}, volume = {41}, number = {4}, year = {2002}, pages = {321--359}, issn = {0933-5846}, mrnumber = {1906504}, mrclass = {03E04}, doi = {10.1007/s001530100101}, note = {\href{https://arxiv.org/abs/math/9807177}{arXiv: math/9807177}}, arxiv_number = {math/9807177} }