I was doing work things and I end up seeing math paper that said something retarded function something. Seconds later wait WHAT function! So yeah according to Introduction to functional differential equations by Jack K. Hale, and Sjoerd M. Verduyn Lunel retarded differential function is: Functions and output vectors and seems to have to be from Banach space of continues functions and there is some other (possible?) limitation around and . Fact that this is called retarded makes me so happy. Name may come from fact that argument of the function is retarded? Couple papers referred to retarded argument but I didn't find definition. Introduction to functional differential equations doesn't reference it so... No idea... Book is interesting though. Have to add to reading pile.