Wave functions and their exp(-ipx) are base of quantum mechanics and QFT. But how to derive them the most simply that we will understand why they are base of QM? It is interesting that Feynman in his book "QED: The Strange Theory of Light and Matter" very simplifies QED, but he did not give a reason how this exp(-ipx) (or sin(px)) arises. It is also interesting that his amplitudes succesively multiplies, but paralelly they suming. But one of the simplest form of multiplicaton function is exp(i..) function. Has this any deeper meaning?