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Measured creatures

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Abstract

We prove that two basic questions on outer measure are undecidable. First we show that consistently every sup-measurable functionf: ℝ2 → ℝ is measurable. The interest in sup-measurable functions comes from differential equations and the question for which functionsf: ℝ2 → ℝ the Cauchy problemy′=f(x,y), y(x0)=y0 has a unique almost-everywhere solution in the classAC t(ℝ) of locally absolutely continuous functions on ℝ. Next we prove that consistently every functionf: ℝ → ℝ is continuous on some set of positive outer Lebesgue measure. This says that in a strong sense the family of continuous functions (from the reals to the reals) is dense in the space of arbitrary such functions.

For the proofs we discover and investigate a new family of nicely definable forcing notions (so indirectly we deal with nice ideals of subsets of the reals—the two classical ones being the ideal of null sets and the ideal of meagre ones).

Concerning the method, i.e., the development of a family of forcing notions, the point is that whereas there are many such objects close to the Cohen forcing (corresponding to the ideal of meagre sets), little has been known on the existence of relatives of the random real forcing (corresponding to the ideal of null sets), and we look exactly at such forcing notions.

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Author information

Authors and Affiliations

  1. Department of Mathematics, University of Nebraska at Omaha, 68182-0243, Omaha, NE, USA

    Andrzej Rosłanowski

  2. Mathematical Institute of Wroclaw University, 50384, Wroclaw, Poland

    Andrzej Rosłanowski

  3. Institute of Mathematics, The Hebrew University of Jerusalem Givat Ram, 91904, Jerusalem, Israel

    Saharon Shelah

  4. Department of Mathematics, Rutgers University, 08854, New Brunswick, NJ, USA

    Saharon Shelah

Authors
  1. Andrzej Rosłanowski
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  2. Saharon Shelah
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Additional information

The first author thanks The Hebrew University of Jerusalem for support during his visits to Jerusalem and the KBN (Polish Committee of Scientific Research) for partial support through grant 2P03A03114.

The research of the second author was partially supported by the Israel Science Foundation. Publication 736.

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Rosłanowski, A., Shelah, S. Measured creatures. Isr. J. Math. 151, 61–110 (2006). https://doi.org/10.1007/BF02777356

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