▁because ▁differential ▁calculus ▁has ▁always ▁been ▁used ▁in ▁gradu ate ▁training ▁and ▁in ▁applications . ▁Moreover , ▁differential ▁calculus ▁has ▁returned ▁to ▁the ▁highest ▁levels ▁of ▁mathematical ▁econom ics , ▁general ▁equilibrium ▁theory ▁( GET ), ▁as ▁pract iced ▁by ▁the ▁" GET - set " ▁( the ▁hum orous ▁design ation ▁due ▁to ▁Jacques ▁H . ▁Dr è ze ). ▁In ▁the ▁ 1 9 6 0 s ▁and ▁ 1 9 7 0 s , ▁however , ▁G ér ard ▁Deb reu ▁and ▁Stephen ▁S male ▁led ▁a ▁rev ival ▁of ▁the ▁use ▁of ▁differential ▁calculus ▁in ▁mathematical ▁econom ics . ▁In ▁particular , ▁they ▁were ▁able ▁to ▁prove ▁the ▁existence ▁of ▁a ▁general ▁equilibrium , ▁where ▁earlier ▁writers ▁had ▁failed , ▁because ▁of ▁their ▁novel ▁mathematics : ▁B aire ▁category ▁from ▁general ▁topology ▁and ▁S ard ' s ▁lemma ▁from ▁differential ▁topology . ▁Other ▁econom ists ▁associated ▁with ▁the ▁use ▁of ▁differential ▁analysis ▁include ▁Eg bert ▁D ier ker , ▁And reu ▁Mas - C ole ll , ▁and ▁Y ves ▁Bal ask o . ▁These ▁adv ances ▁have ▁changed ▁the ▁traditional ▁narr ative ▁of ▁the ▁history ▁of ▁mathematical ▁econom ics , ▁following ▁von ▁Ne umann , ▁which ▁celebrated ▁the ▁abandon ment ▁of ▁differential ▁calculus . ▁ ▁Game ▁theory ▁ ▁John ▁von ▁Ne umann , ▁working ▁with ▁O sk ar ▁M orgen st ern ▁on ▁the ▁theory ▁of ▁games , ▁broke ▁new ▁mathematical ▁ground ▁in ▁ 1 9 4 4 ▁by ▁extending ▁functional ▁analyt ic ▁methods ▁related ▁to ▁convex ▁sets