The Bookseller.com reports HarperCollins has acquired
The book had originally been scheduled for publication [in the UK] by Gollancz, but the rights recently became available. The book will be published later this year in hardback and as a deluxe collectors’ edition. HC also plans to relaunch its Clarke backlist.
(In an interview with Clarke the spelling of “theorem” was different than used by Thebookseller — theorum — and at this writing I don’t know whether that is a typo or just a difference in U.S. and U.K. spellings.)
[Thanks to Andrew Porter for the link.]
It isn’t a UK spelling.