If you're building a 3D model and you want to connect a square and a circle, but you want that 3D model to be smooth, this might help you.

The formula for this shape is (|x|^(1/z)+|y|^(1/z))^z≤r for r=0.25, x and y from -0.25 to 0.25 and z from 0 to 0.5.