Continuous Time
Periodic signals
A periodic signal is one that for a given real number "a": $ x(t) = x(t+a) $
http://en.wikibooks.org/wiki/Signals_and_Systems/Periodic_Signals
Non Periodic signals
Non-periodic signals are those which no value of "a" satisfies the following equation. $ x(t) = x(t+a) $
http://ac3filter.net/files/docs/ac3filter_1_30b/loudness_eng.html
