I am sorry if this seems to be a repeated question that appeared on stackexchange, but I don't think the reply was satisfying to me and so I decided to ask in an independent post.
BCH formula is a beautiful formula, but following "On the Convergence of Exponential Operators—the Zassenhaus Formula, BCH Formula and Systematic Approximants" by Masuo Suzuki,
The equation holds only if the concerned self-adjoint operators are bounded.
I would like to ask is there an analogous formula for exponent of unbounded operators defined with spectral theorem of unbounded operator? It will be the best if I can have some reference.
Thanks in advance for your answering.