), ▁Paul ▁Samuel son ▁identified ▁a ▁common ▁parad ig m ▁and ▁mathematical ▁structure ▁across ▁multiple ▁fields ▁in ▁the ▁subject , ▁building ▁on ▁previous ▁work ▁by ▁Alfred ▁Marshall . ▁Found ations ▁took ▁mathematical ▁concepts ▁from ▁ ▁physics ▁and ▁applied ▁them ▁to ▁economic ▁problems . ▁This ▁broad ▁view ▁( for ▁example , ▁comparing ▁Le ▁Ch atel ier ' s ▁principle ▁to ▁t â ton nement ) ▁drives ▁the ▁fundamental ▁prem ise ▁of ▁mathematical ▁econom ics : ▁systems ▁of ▁economic ▁actors ▁may ▁be ▁mode led ▁and ▁their ▁behavior ▁described ▁much ▁like ▁any ▁other ▁system . ▁This ▁extension ▁followed ▁on ▁the ▁work ▁of ▁the ▁marg inal ists ▁in ▁the ▁previous ▁century ▁and ▁extended ▁it ▁significantly . ▁Samuel son ▁approached ▁the ▁problems ▁of ▁applying ▁individual ▁utility ▁maxim ization ▁over ▁aggregate ▁groups ▁with ▁compar ative ▁stat ics , ▁which ▁comp ares ▁two ▁different ▁equilibrium ▁states ▁after ▁an ▁ex ogen ous ▁change ▁in ▁a ▁variable . ▁This ▁and ▁other ▁methods ▁in ▁the ▁book ▁provided ▁the ▁foundation ▁for ▁mathematical ▁econom ics ▁in ▁the ▁ 2 0 th ▁century . ▁ ▁Linear ▁models ▁ ▁Rest rict ed ▁models ▁of ▁general ▁equilibrium ▁were ▁form ulated ▁ ▁by ▁John ▁von ▁Ne umann ▁in ▁ 1 9 3 7 . ▁Un like ▁earlier ▁versions , ▁the ▁models ▁of ▁von ▁Ne umann ▁had ▁inequality ▁constraints . ▁For ▁his ▁model ▁of ▁an ▁expand ing ▁economy , ▁von ▁Ne umann ▁proved ▁the ▁existence ▁and ▁uniqu eness ▁of ▁an ▁equilibrium ▁using ▁his ▁general ization ▁of ▁Br ou wer ' s ▁fixed ▁point ▁theorem . ▁Von ▁Ne umann ' s