The "efficiency wage" for nutrition models (Dasgupta-Ray) require solving for non-convexities. The manual explains how to prove multiple equilibria (poverty trap vs. high-income equilibrium) using the e(w) function.
If you find a PDF titled "Debraj Ray Solutions Manual Full.pdf" on a file-sharing site, be warned: it is almost certainly unofficial, often contains errors, and is usually incomplete. Development Economics Debraj Ray Solutions Manual
Here is the hidden curriculum of Ray’s problem sets: often contains errors
Are you an instructor with access to the official manual? Consider sharing your review of it on academic forums to help students distinguish between accurate and pirated copies. Are you a student? Visit your university library’s reserve desk—sometimes, the previous TA’s binder is hiding in plain sight. Development Economics Debraj Ray Solutions Manual