Recently I learned some communication theory, which uses negative frequency, what does negative frequency means? So I checked text book, found the negative frequency comes from the Fourier transform, which comes from the euler formula, e-jx=cosx+isinx, or another form cosx=(ejx+e-jx)/2, then any physical quantity can be expressed in negative part and positive part. Can anyone explain this (negative frequency) further? (In physics, I can not understand this, so this is just a expression). Some communication theory use this much, like AM, DSB,modulation method. Does this technical develop after mathematical derivation(like the electromagnetic wave) or mathematical is latter developed after experiments. Are there any good text book introduce this (communication theory and technology and their history) clearly?