Wiki
Wiki

Research notes on every problem, and a library of the papers behind them. Built from the open erdos repository.

Updated


Claim. Let c=λ\mathfrak c=\lambda with $\operatorname{cf}(\lambda)> \omega_1$, and add ω1\omega_1 Cohen reals. In the extension, every family of c\mathfrak c pairwise distinct entire functions has a point zz at which it takes c\mathfrak c distinct values. In particular, starting from c=ℵ2\mathfrak c=\aleph_2, the extension has c=ℵ2\mathfrak c=\aleph_2 and every family of entire functions taking at most ℵ1\aleph_1 values at each point has at most ℵ1\aleph_1 members, so the question of Problem 1119 has a positive answer there for its only admissible cardinal, m=ℵ1\mathfrak m=\aleph_1. This is Theorem 2.1 of the preprint Sh:1078 in Shelah's archive (version of 2017-01-05), which dates the page; the paper was published as Ashutosh Kumar and Saharon Shelah, On a question about families of entire functions, Fund. Math. 239 (2017), no. 3, 279--288, and the theorem numbering of the published version was not compared. The source card records the statements.

Covers. The case m+=c\mathfrak m^+=\mathfrak c with the continuum hypothesis failing: in the model of Theorem 2.1 built from c=ℵ2\mathfrak c=\aleph_2, the cardinal m=ℵ1\mathfrak m=\aleph_1 has m+=c\mathfrak m^+=\mathfrak c and the answer is yes. For the literal statement, non-refutability needs no such model: under the continuum hypothesis no cardinal m\mathfrak m satisfies ℵ0<m<c\aleph_0<\mathfrak m<\mathfrak c, so the statement holds vacuously in every model of CH, such as Gödel's constructible universe, and whenever m+<c\mathfrak m^+<\mathfrak c the answer is yes by Erdős's counting argument. What this theorem adds is a positive answer in a model of the negation of CH in the hard case m+=c\mathfrak m^+=\mathfrak c. Independence of that case needs both this model and Schilhan and Weinert's result, a model with c=ℵ2\mathfrak c=\aleph_2 in which the answer for m=ℵ1\mathfrak m=\aleph_1 is no. The paper's Theorem 3.1 also gives a model of the negation of the continuum hypothesis with a family of c\mathfrak c entire functions taking fewer than c\mathfrak c values at every point, but there c=ℵω1\mathfrak c=\aleph_{\omega_1} is singular and the value sets have no uniform bound below c\mathfrak c, so that theorem does not answer the problem's question for a single m\mathfrak m.

Argument, in outline. A Cohen-generic point zz avoids every meager set coded before it, and two distinct entire functions agree on at most a countable set; the authors use both to show that c\mathfrak c distinct functions cannot all take few values at zz. The proof is not reconstructed on this page.

Context. Erdős showed in 1964 (source card) that the countable case depends on the continuum hypothesis, and his counting argument gives a positive answer whenever $\mathfrak m^+<\mathfrak c$, which Hayman's 1974 problem list calls easy; the open case is m+=c\mathfrak m^+=\mathfrak c, which this model realizes with m=ℵ1\mathfrak m=\aleph_1 and c=ℵ2\mathfrak c=\aleph_2.

Acceptance. The result appeared in a refereed journal, Fundamenta Mathematicae, in 2017, the refereed evidence; the preprint was not compared with the published version. The site's curator, Thomas Bloom, marks the problem INDEPENDENT and records in the commentary that Kumar and Shelah gave a model with c=ℵ2\mathfrak c=\aleph_2 in which the answer is yes for m=ℵ1\mathfrak m=\aleph_1: that curator credit is the reviewed evidence. No formalization of this result is known.