Yes, in general statements like this are theoretically possible to prove, but not remotely practical. There might be some specific (A,B,LLM) triples for which you can prove such a statement but I expect that none of these are generalizable to actually useful statements.
No GPT-style architecture is (in itself) capable of truly universal computation, but in practice functions they can implement are far beyond our ability to adequately analyze.
Yes, in general statements like this are theoretically possible to prove, but not remotely practical. There might be some specific (A,B,LLM) triples for which you can prove such a statement but I expect that none of these are generalizable to actually useful statements.
No GPT-style architecture is (in itself) capable of truly universal computation, but in practice functions they can implement are far beyond our ability to adequately analyze.