=== L i n e a r T i m e I n v a r i a n t S y s t e m s ( L T I S y s t e m s ) === A l i n e a r t i m e i n v a r i a n t s y s t e m i s o n e t h a t i s l i n e a r ( s u p e r p o s i t i o n a n d p r o p o r t i o n a l i t y a p p l y ) a n d o n e t h a t d o e s n ′ t c h a n g e w i t h t i m e . F o r e x a m p l e a c i r c u i t w i t h f i x e d c a p a c i t o r s , r e s i s t o r s , a n d i n d u c t o r s h a v i n g a n i n p u t a n d a n o u t p u t i s l i n e a r a n d t i m e i n v a r i a n t . I f a c a p a c i t o r c h a n g e d v a l u e w i t h t i m e , t h e n i t w o u l d n o t b e t i m e i n v a r i a n t . === E i g e n f u n c t i o n s a n d E i g e n v a l u e s o f a n L T I S y s t e m s === I t i s a n i n t e r e s t i n g e x e r c i s e t o s h o w t h a t < m a t h > e j ω t {\displaystyle ===LinearTimeInvariantSystems(LTISystems)===Alineartimeinvariantsystemisonethatislinear(superpositionandproportionalityapply)andonethatdoesn'tchangewithtime.Forexampleacircuitwithfixedcapacitors,resistors,andinductorshavinganinputandanoutputislinearandtimeinvariant.Ifacapacitorchangedvaluewithtime,thenitwouldnotbetimeinvariant.===EigenfunctionsandEigenvaluesofanLTISystems===Itisaninterestingexercisetoshowthat<math>e^{j\omega t}} are eigenfunctions of any LTI system. The eigenvalues are ω {\displaystyle \omega } .
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