Israel Journal of Mathematics

, Volume 151, Issue 1, pp 61–110 | Cite as

Measured creatures

  • Andrzej Rosłanowski
  • Saharon Shelah


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 classACt(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.


Borel Function Outer Measure Force Notion Positive Lebesgue Measure Finite Tree 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© The Hebrew University Magnes Press 2006

Authors and Affiliations

  • Andrzej Rosłanowski
    • 1
    • 2
  • Saharon Shelah
    • 3
    • 4
  1. 1.Department of MathematicsUniversity of Nebraska at OmahaOmahaUSA
  2. 2.Mathematical Institute of Wroclaw UniversityWroclawPoland
  3. 3.Institute of MathematicsThe Hebrew University of Jerusalem Givat RamJerusalemIsrael
  4. 4.Department of MathematicsRutgers UniversityNew BrunswickUSA

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