General ⇒ Chapter-Section-Number referencing

[mt226]

I'm writing a file in amslatex, documentstyle amsbook.

Say I have Thm. 3 in Sec. 2 of Ch.1. Inside Sec. 2, I want it referred to as Thm. 1. In Ch. 1, but outside Sec. 2, I want it referred to as Thm. 2.1. Finally, outside Ch. 1 it should be Thm. 1.2.3

How do I get latex to do this automatically?

Thanks,
Mak