June | 2015 | A kind of library

Update

June 29, 2015

So, we moved to Ann Arbor.

We visited the Hands-On Museum (thanks to Zach and Diana, and congratulations!),

and attended the Math Reviews Summer picnic.

I started today.

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3 Comments | Life | Tagged: Francisco, Isabel | Permalink
Posted by andrescaicedo

You are currently browsing the A kind of library blog archives for June, 2015.
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- Answer by Andrés E. Caicedo for "Inner Regularity" of probability measure on totally ordered sets October 7, 2019
  Consider a subset $\Omega$ of $\mathbb R$ of size $\aleph_1$ and ordered in type $\omega_1$. (This uses the axiom of choice.) Let $\mathcal F$ be the $\sigma$-algebra generated by the initial segments of $\Omega$ under the well-ordering (so all sets in $\mathcal F$ are countable or co-countable), with the measure that assigns $0$ to the countable sets and $1 […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Question about Jech's proof of V = L implies GCH June 3, 2019
  You assume $\omega_\alpha\subseteq M$ and $X\in M$ so that $X$ belongs to the transitive collapse of $M$ (because if $\pi$ is the collapsing map, $\pi(X)=\pi[X]=X$. You assume $|M|=\aleph_\alpha$ so that the transitive collapse of $M$ has size $\aleph_\alpha$. Since you also have that this transitive collapse is of the form $L_\beta$ for some $\beta$, it fol […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Smallest $\beta$ such that it is provable that $2^{\aleph_\beta} > 2^{\aleph_0}$ January 16, 2019
  Perhaps the following may clarify the comments: for any ordinal $\delta$, there is a Boolean-valued extension of the universe of sets where $2^{\aleph_0}>\aleph_\delta$ holds. If you rather talk of models than Boolean-valued extensions, what this says is that we can force while preserving all ordinals, and in fact all initial ordinals, and make the contin […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Arguments against large cardinals October 29, 2010
  I do not know of any active set theorists who think large cardinals are inconsistent. At least, within the realm of cardinals we have seriously studied. [Reinhardt suggested an ultimate axiom of the form "there is a non-trivial elementary embedding $j:V\to V$". Though some serious set theorists found it of possible interest immediately following it […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Nonessential use of large cardinals December 10, 2010
  There is a fantastic (and not too well-known) result of Shelah stating that $L({\mathcal P}(\lambda))$ is a model of choice whenever $\lambda$ is a singular strong limit of uncountable cofinality. This is a consequence of a more general theorem that can be found in 4.6/6.7 of "Set Theory without choice: not everything on cofinality is possible", Ar […]
  Andrés E. Caicedo
Math.StackExchange
- Answer by Andrés E. Caicedo for Can we find a set $B⊂ℕ$ such that an equivalence holds true October 9, 2019
  Let $B=\{n\mid \forall m\in A\,(n>m)\}$. That is, $B$ is the collection of natural numbers that are larger than all elements of $A$. If $A$ is infinite, $B$ is empty. If $A$ is finite, $B$ is not only infinite, but in fact it is a tail of the natural numbers; more precisely, it is the set of all natural numbers strictly larger than the maximum of $A$.
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Exists infinitely many as a numerical-quantifier October 6, 2019
  The standard notation in logic would be $\exists^\infty$. The exclamation mark ! is used to indicate uniqueness, $\exists^{!n} x\,\phi(x)$ being "there are exactly $n$ distinct elements $x$ such that $\phi(x)$". So, the standard reading of $\exists^{!\infty}x\,\phi(x)$ would be "there are exactly infinitely many $x$ such that..." which is […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Does there exist a sequence that has countably infinite convergent subsequences? August 21, 2019
  Take $a_n=p$, where $p$ is the smallest prime dividing $n$. If a subsequence converges, it converges to a prime $p$, in which case except for finitely many initial terms, the sequence is eventually constant with value $p$. But the number of initial terms is arbitrary.
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for What is the consistency strength of generalized failure of the continuum hypothesis. April 14, 2019
  The precise consistency strength of the global failure of the generalized continuum hypothesis is somewhat technical to state. As far as I know, it has not been published, but I think we have a decent understanding of what the correct statement should be. The most relevant paper towards this result is MR2224051 (2007d:03082). Gitik, Moti Merimovich, Carmi. P […]
  Andrés E. Caicedo
- Answer by Andrés E. Caicedo for Is "integrability" equivalent to "having antiderivative"? March 9, 2014
  There are integrable functions that are not derivatives: Any function that is continuous except at a single point, where it has a jump discontinuity, is an example. (Derivatives have the intermediate value property.) More interestingly, we can ask whether the existence of an antiderivative ensures integrability. The answer depends on what integral you are co […]
  Andrés E. Caicedo
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