▁ 1 9 1 2 ▁Cart an ▁became ▁Professor ▁there , ▁based ▁on ▁the ▁reference ▁he ▁received ▁from ▁Po inc ar é . ▁He ▁remained ▁in ▁Sor bon ne ▁until ▁his ▁ret irement ▁in ▁ 1 9 4 0 ▁and ▁spent ▁the ▁last ▁years ▁of ▁his ▁life ▁teaching ▁mathematics ▁at ▁the ▁É cole ▁Norm ale ▁Sup érieure ▁for ▁girls . ▁ ▁As ▁a ▁student ▁of ▁Cart an , ▁the ▁ge ometer ▁Sh i ing - Sh en ▁Ch ern ▁wrote : Us ually ▁the ▁day ▁after ▁[ me eting ▁with ▁Cart an ] ▁I ▁would ▁get ▁a ▁letter ▁from ▁him . ▁He ▁would ▁say , ▁“ After ▁you ▁left , ▁I ▁thought ▁more ▁about ▁your ▁questions ... ” — he ▁had ▁some ▁results , ▁and ▁some ▁more ▁questions , ▁and ▁so ▁on . ▁He ▁knew ▁all ▁these ▁papers ▁on ▁simple ▁Lie ▁groups , ▁Lie ▁al gebras , ▁all ▁by ▁heart . ▁When ▁you ▁saw ▁him ▁on ▁the ▁street , ▁when ▁a ▁certain ▁issue ▁would ▁come ▁up , ▁he ▁would ▁pull ▁out ▁some ▁old ▁en velope ▁and ▁write ▁something ▁and ▁give ▁you ▁the ▁answer . ▁And ▁sometimes ▁it ▁took ▁me ▁hours ▁or ▁even ▁days ▁to ▁get ▁the ▁same ▁answer ... ▁I ▁had ▁to ▁work ▁very ▁hard . In ▁ 1 9 2 1 ▁he ▁became ▁a ▁foreign ▁member ▁of ▁the ▁Polish ▁Academy ▁of ▁Learning ▁and ▁in ▁ 1 9 3 7 ▁a ▁foreign ▁member ▁of ▁the ▁Royal ▁Netherlands ▁Academy ▁of ▁Arts ▁and ▁Sciences . ▁In ▁ 1 9 3 8 ▁he ▁participated ▁in ▁the