Indicator function question


I have a question about indicator functions. I have a variable b_t in the model which can denote borrowing, if positive, and retained earnings if negative. The saving rate is r_st=r_ft-k, wher r_ft is the borrowing rate. I have that:
image, where image.

I am not sure if this is modelled as a standard indicator function? Thank you for your help.

Yes, an indicator function like that would work.

Thank you!