Midterm , clarifying comments and …

Midterm, clarifying comments and errata:

• 1.: The magnetic field is a constant and is not time-dependent.
• 1.1: Replace "eigenfunctions" with "eigenstates".
• 2.2: You do NOT need to show [b,b]=0 and [b^+,b^+]=0. These are obviously true regardless of how b is expressed in terms of the a's. The info [a,a]=0 below H is also superfluous.