Full Idea
The expression 'the least rapidly convergent series' has a sense but demonstrably there is no reference, since a less rapidly convergent series (for any given series) can always be found.
Gist of Idea
In maths, there are phrases with a clear sense, but no actual reference
Source
Gottlob Frege (On Sense and Reference [1892], p.28)
Book Reference
Frege,Gottlob: 'Translations from the Writings of Gottlob Frege', ed/tr. Geach,P/Black,M [Blackwell 1980], p.58
A Reaction
A nice example. 'The second Kennedy assassin' has a clear meaning, but does it have a reference? The meaning 'points at' a possible reference. We yet discover an identity.