Souslin's theorem

Souslin's theorem

[′sü‚slanz thir·əm]
(mathematics)
The theorem that, if both a subset of a separable, complete metric space and its complement in this space are continuous images of Borel sets in this space, then the subset is itself a Borel set.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.