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Separate approximate continuity implies measurability

Published online by Cambridge University Press:  24 October 2008

Roy O. Davies
Affiliation:
The University, Leicester

Extract

It is known that a real-valued function f of two real variables which is continuous in each variable separately need not be continuous in (x, y), but must be in the first Baire class (1). Moreover if f is continuous in x for each y and merely measurable in y for each x then it must be Lebesgue-measurable (7), and this result can be extended to more general product spaces (2). However, the continuum hypothesis implies that this result fails if continuity is replaced by approximate continuity, as can be seen from the proof of Theorem 2 of (2). This makes Mišik's question (5) very natural: is a function which is separately approximately continuous in both variables necessarily Lebesgue-measurable? Our main aim is to establish an affirmative answer. It will be shown that such a function must in fact be in the second Baire class, although not necessarily in the first Baire class (unlike approximately continuous functions of one variable (3)). Finally, we show that the existence of a measurable cardinal would imply that a separately continuous real function on a product of two topological finite complete measure spaces need not be product-measurable.

Type
Research Article
Copyright
Copyright © Cambridge Philosophical Society 1973

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References

REFERENCES

(1)Baire, R.Sur les functions do variables réelles. Annali di Mat., (3) 3 (1899), 1123.CrossRefGoogle Scholar
(2)Davies, R. O. and Dravecký, J.On the measurability of functions of two variables. Mat. Càs. Slov. Akad. Vied. (forthcoming).Google Scholar
(3)Denjoy, A.Sur les fonctions dérivées sommables. Bull. Soc. Math. France 43 (1915), 161248.CrossRefGoogle Scholar
(4)Goffman, C., Neugebauer, C. J. and Nishiura, T.Density topology and approximate continuity. Duke Math. J. 28 (1961), 497505.CrossRefGoogle Scholar
(5)Lipiński, J. S.On measurability of functions of two variables. Bull. Acad. Polon. Sci. Sér. Sci. Math. Astr. Phys. 20, (1972), 131135.Google Scholar
(6)Saks, S.Theory of the integral (Warsaw, 1937).Google Scholar
(7)Ursell, H. D.Some methods of proving measurability. Fund. Math. 32 (1939), 311330.CrossRefGoogle Scholar
(8)Zahorski, Z.Sur la première dérivée. Trans. Amer. Math. Soc. 69 (1950), 154.Google Scholar