Blumberg theorem

In mathematics, the Blumberg theorem states that for any real function $f:\mathbb {R} \rightarrow \mathbb {R}$ there is a dense subset D of $\mathbb {R}$ such that the restriction of f to D is continuous.

For instance, the restriction of the Dirichlet function (the indicator function of the rational numbers $\mathbb {Q}$ ) to $\mathbb {Q}$ is continuous, although the Dirichlet function is nowhere continuous.

Blumberg spaces

More generally, a Blumberg space is a topological space X for which any function $f:X\rightarrow \mathbb {R}$ admits a continuous restriction on a dense subset of X. Blumberg theorem therefore asserts that $\mathbb {R}$ (equipped with its usual topology) is a Blumberg space.

If X is a metric space, then X is a Blumberg space if and only if it is a Baire space.

References

Blumberg, Henry (1922). "New properties of all real functions" (PDF). Proceedings of the National Academy of Sciences. 8 (1): 283-288.
Blumberg, Henry (1922). "New properties of all real functions". Transactions of the American Mathematical Society. 24: 113-128.
Bradford, J. C.; Goffman, Casper (1960). "Metric spaces in which Blumberg's theorem holds". Proceedings of the American Mathematical Society. 11: 667-670.
White, H. E. (1974). "Topological spaces in which Blumberg's theorem holds". Proceedings of the American Mathematical Society. 44: 454-462.
https://www.encyclopediaofmath.org/index.php/Blumberg_theorem

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