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

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

If ${\displaystyle X}$  is a metric space then ${\displaystyle X}$  is a Blumberg space if and only if it is a Baire space.^[1] The Blumberg problem is to determine whether a compact Hausdorff space must be Blumberg. A counterexample was given in 1974 by Ronnie Levy, conditional on Luzin's hypothesis, that ${\displaystyle 2^{\aleph _{0}}=2^{\aleph _{1}}.}$ ^[2] The problem was resolved in 1975 by William A. R. Weiss, who gave an unconditional counterexample. It was constructed by taking the disjoint union of two compact Hausdorff spaces, one of which could be proven to be non-Blumberg if the Continuum Hypothesis was true, the other if it was false.^[3]

The restriction of any continuous function to any subset of its domain (dense or otherwise) is always continuous, so the conclusion of the Blumberg theorem is only interesting for functions that are not continuous. Given a function that is not continuous, it is typically not surprising to discover that its restriction to some subset is once again not continuous,^{[note 1]} and so only those restrictions that are continuous are (potentially) interesting. Such restrictions are not all interesting, however. For example, the restriction of any function (even one as interesting as the Dirichlet function) to any subset on which it is constant will be continuous, although this fact is as uninteresting as constant functions. Similarly uninteresting, the restriction of any function (continuous or not) to a single point or to any finite subset of ${\displaystyle \mathbb {R} }$  (or more generally, to any discrete subspace of ${\displaystyle \mathbb {R} ,}$  such as the integers ${\displaystyle \mathbb {Z} }$ ) will be continuous.

One case that is considerably more interesting is that of a non-continuous function ${\displaystyle f}$  whose restriction to some dense subset ${\displaystyle D}$  (of its domain) is continuous. An important fact about continuous ${\displaystyle \mathbb {R} }$ -valued functions defined on dense subsets is that a continuous extension to all of ${\displaystyle \mathbb {R} ,}$  if one exists, will be unique (there exist continuous functions defined on dense subsets of ${\displaystyle \mathbb {R} ,}$  such as ${\displaystyle f(x)=1/x,}$  that cannot be continuously extended to all of ${\displaystyle \mathbb {R} }$ ).

Thomae's function, for example, is not continuous (in fact, it is discontinuous at every rational number) although its restriction to the dense subset ${\displaystyle \mathbb {R} \setminus \mathbb {Q} }$  of irrational numbers is continuous. Similarly, every additive function ${\displaystyle \mathbb {R} \to \mathbb {R} }$  that is not linear (that is, not of the form ${\displaystyle x\mapsto cx}$  for some constant ${\displaystyle c\in \mathbb {R} }$ ) is a nowhere continuous function whose restriction to ${\displaystyle \mathbb {Q} }$  is continuous (such functions are the non-trivial solutions to Cauchy's functional equation). This raises the question: can such a dense subset always be found? The Blumberg theorem answer this question in the affirmative. In other words, every function ${\displaystyle \mathbb {R} \to \mathbb {R} }$  − no matter how poorly behaved it may be − can be restricted to some dense subset on which it is continuous. Said differently, the Blumberg theorem shows that there does not exist a function ${\displaystyle \mathbb {R} \to \mathbb {R} }$  that is so poorly behaved (with respect to continuity) that all of its restrictions to all possible dense subsets are discontinuous.

The theorem's conclusion becomes more interesting as the function becomes more pathological or poorly behaved. Imagine, for instance, defining a function ${\displaystyle f:\mathbb {R} \to \mathbb {R} }$  by picking each value ${\displaystyle f(x)}$  completely at random (so its graph would be appear as infinitely many points scattered randomly about the plane ${\displaystyle \mathbb {R} ^{2}}$ ); no matter how you ended up imagining it, the Blumberg theorem guarantees that even this function has some dense subset on which its restriction is continuous.

• Closed graph theorem (functional analysis) – Theorems connecting continuity to closure of graphs
• Densely defined operator – Function that is defined almost everywhere (mathematics)
• Hahn–Banach theorem – Theorem on extension of bounded linear functionals
• Tietze extension theorem – Continuous maps on a closed subset of a normal space can be extended
• Whitney extension theorem – Partial converse of Taylor's theorem

• Blumberg, Henry (1922). "New properties of all real functions" (PDF). Proceedings of the National Academy of Sciences. 8 (1): 283–288. doi:10.1073/pnas.8.10.283. PMC 1085149. PMID 16586898.
• Blumberg, Henry (September 1922). "New properties of all real functions". Transactions of the American Mathematical Society. 24 (2): 113–128. doi:10.1090/S0002-9947-1922-1501216-9. JSTOR 1989037.
• Bradford, J. C.; Goffman, Casper (1960). "Metric spaces in which Blumberg's theorem holds". Proceedings of the American Mathematical Society. 11: 667–670.
• "Variations on Blumberg's Theorem", Jack B. Brown, Real Analysis Exchange 9, #1 (1983/1984), pp. 123–137, doi:10.2307/44153521, JSTOR 44153521.
• "'Big' Continuous Restrictions of Arbitrary Functions", K. C. Ciesielski, M. E. Martínez-Gómez and J. B. Seoane-Sepúlveda, The American Mathematical Monthly, 126, #6 (June–July 2019), pp. 547–552, JSTOR 48661187.
• "Strongly non-Blumberg spaces", Ronnie Levy, General Topology and its Applications, 4, #2 (June 1974), pp. 173–177, doi:10.1016/0016-660X(74)90019-1.
• "A solution to the Blumberg problem", William A. R. Weiss, Bulletin of the American Mathematical Society 81, #5 (September 1975), pp. 957–958, doi:10.1090/S0002-9904-1975-13914-0.
• "The Blumberg problem", William A. R. Weiss, Transactions of the American Mathematical Society 230 (June 1977), pp. 71–85, doi:10.2307/1997712, JSTOR 1997712.
• White, H. E. (1974). "Topological spaces in which Blumberg's theorem holds". Proceedings of the American Mathematical Society. 44: 454–462.
• "Blumberg theorem", Encyclopedia of Mathematics, EMS Press, 2001 [1994]