Phragmen–Brouwer theorem

In topology, the Phragmén–Brouwer theorem, introduced by Lars Edvard Phragmén and Luitzen Egbertus Jan Brouwer, states that if X is a normal connected locally connected topological space, then the following two properties are equivalent:

• If A and B are disjoint closed subsets whose union separates X, then either A or B separates X.
• X is unicoherent, meaning that if X is the union of two closed connected subsets, then their intersection is connected or empty.

The theorem remains true with the weaker condition that A and B be separated.

References

• R.F. Dickman jr (1984), "A Strong Form of the Phragmen–Brouwer Theorem", Proceedings of the American Mathematical Society 90 (2): 333–337, doi:10.2307/2045367 
• Hunt, J.H.V. (1974), "The Phragmen-Brouwer theorem for separated sets", Bol. Soc. Mat. Mex., II. Ser. 19: 26–35, Zbl 0337.54021 
• Wilson, W. A. (1930), "On the Phragmén–Brouwer theorem", Bulletin of the American Mathematical Society 36 (2): 111–114, doi:10.1090/S0002-9904-1930-04901-0, ISSN 0002-9904, MR 1561900