Logical constraints

[zombie]

I am quite new to GAMS and would appreciate your help. I have two variables a and b. I want b to be 0 when a is greater than or equal to 0.5. Otherwise, b is to be 1. I am trying to solve it as an NLP problem so, I can't declare b as a binary variable. How do I then formulate the constraints for this?

[bussieck]

There are plenty of questions like this already in the forum. Just search for logic expression. -Michael