Mathematical symbol is "relative compliment"?

Question

A bit of a dumb question, but it's blocking my work at the moment so I'm hoping it will be tolerated !

I have a research paper describing an approach to Convex Quadratic problems (D. GoldFarb and A. Idnani 20 Oct. 1981).

Unfortunately it's a scanned copy (I bought online and downloaded) and I cannot quite read one of the symbols for sure (I am not an expert on this sort of maths).

I wander if anyone can confirm if this symbol is "Relative compliment" ? Which is what I suspect. I have circled the symbol in red:

Answer 1

Yes, the complement of a set is written with the symbol $\setminus$ :

$$B\setminus A=\{x \in B| x \not\in A\}.$$

See here for details.

Answer 2

Yes, the idea is that in b case A becomes smaller and smaller (hence K-A becomes bigger and bigger) until all constraints are satisfied (a case)