How to rotate a Mathematics.quaternion ?

I find Mathematics.quaternion has a lot fewer functions than UnityEngine.Quaternion. What’s the correct way to rotate a Mathematics.quaternion by an Euler angle (in float3)?

Have you actually checked the documentation of the mathematics package? Also the source code of the quaternion struct is also available online so you can see what methods it contains. What exact method do you miss that Quaternion has and that the quaternion from Mathematics don’t? Maybe you’re looking for the mul method? It multiplies either twu quaternions with each other or a quaternion with a float3. The whole mathematics library was designed to have a similar syntax to usual shader code.

why you don’t use the dedicated functions provided by unity to do these calculations?

the quaternions are high level math, if you don’t have a degree in mathematics I suggest leaving them alone and use the functions provided by unity