[HoustonHam] Math guru wanted - little OT

[Vaclav]

I am a software geek / hobbyist working on an amateur radio applications using FFT. But I ain't no mathematician and could use some assistance from an expert in USING FFT. I got the transfer from time to frequency domain figured out and working, right now I am trying to implement a filter to get rid of the zero coefficient and low frequency components. That is not that hard just deleting them, but eventually I would like to be able do some passband filtering and other cool stuff. So far I have no clue how to implement transfer from frequency to time domain. 

Sure would like to hear from someone who could give some hints / assistance.
73 Vaclav AA7EJ