1. The problem statement, all variables and given/known data The question is to evaluate the expression e^-iA, where A is a Hermitian operator whose eigenvalues are known (but not given) using bra-ket algebra. 2. Relevant equations See above. 3. The attempt at a solution I have been looking around, reading the textbook and course notes, checking the web and so on. I have learned that an expression in this form is a 'unitary operator', and what that means. I'm pretty comfortable with the bra-ket notation, but I'm struggling just to *get into it*. ;-) If you can help me find a 'way in' to get started, that'd be great: at this point I'm still just kinda staring blankly at it, despite all my efforts so far. Oh, yeah, I overhead another student mention using a Taylor series. I get how to convert the exponential to a Taylor series, but I'm not so sure how it helps... and the other student might be wrong.