Thanks!
Since these are state vectors, would
(|2><1| + |1><2|) * |2>) = |2><1|2> + |1><2|2> = |1> ?
I believe these state vectors are orthogonal, so the <1|2> term is 0, but my textbooks isn't very clear.
For my homework, I have a problem in which a (harmonic oscillator) system is prepared in state n=2 for t<0.
For time t>0, there is a perturbation given by
V(t) = sqrt(3/4)*h_bar*omega* (|2><1| + |1><2|)
After this I need to compute the probability amplitudes. However, my background...