In graduate school, is it possible to enroll in courses even though one is not doing their phd in that subject ? So can a math phd student take physics graduate courses ? Also, does the math phd student have to formally satisfy the prerequisites for that physics course. Let's say he/she wants to take a course on graduate quantum mechanics, but that student has never formally taken an undergraduate quantum mechanics but has self-studied it extensively. Would there be any way for that student to get enrolled in that course or will they have to audit it ?