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forcing, large cardinals, descriptive set theory, infinite combinatorics, cardinal characteristics, forcing axioms, ultrapowers, measures, reflection, pcf theory, models of set theory, axioms of set theory, independence, axiom of choice, continuum hypothesis, determinacy, Borel equivalence relations, Boolean-valued models, embeddings, orders, relations, transfinite recursion, set theory as a foundation of mathematics, the philosophy of set theory.

38 votes
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Do there exist non-PIDs in which every countably generated ideal is principal?

No such ring exists. Suppose otherwise. Let $I$ be a non-principal ideal, generated by a collection of elements $f_\alpha$ indexed by the set of ordinals $\alpha<\gamma$ for some $\gamma$. Consid …
Hugh Thomas's user avatar
  • 6,327
3 votes
2 answers
648 views

Can this informal argument (for the fact that almost all reals in the unit interval are irra...

In the textbook from which I am teaching a Discrete Math course, the authors propose randomly generating an infinite sequence of decimal digits $d_1, d_2, \dots$. We are to think of this as the decim …
Hugh Thomas's user avatar
  • 6,327
7 votes

Infinite Partitions of the Primes and Sums of Reciprocals (Revised)

The following is rather more simple-minded that what you are suggesting. Let's say we have a deck of cards, with the cards labelled by the primes in order. We are going to think of constructing th …
Hugh Thomas's user avatar
  • 6,327