▁influ ential ▁French ▁art ▁deal ers ▁of ▁the ▁ 2 0 th ▁century . ▁The ▁son ▁of ▁an ▁ant ique ▁deal er ▁Alexander ▁Ros enberg ▁and ▁brother ▁of ▁the ▁gallery ▁owner ▁Paul ▁Ros enberg ▁( 2 1 ▁rue ▁de ▁la ▁Bo ét ie , ▁Paris ), ▁Lé once , ▁a ▁prominent ▁gallery ▁owner ▁in ▁Paris ▁at ▁the ▁end ▁of ▁World ▁War ▁I , ▁would ▁become ▁one ▁of ▁the ▁world ' s ▁major ▁deal ers ▁of ▁Modern ▁art . ▁ ▁Le aving ▁the ▁family - owned ▁gallery ▁in ▁ 1 9 1 0 ▁Lé once ▁opened ▁his ▁own ▁business ▁called ▁Ha ute ▁E poque ▁at ▁ 1 9 ▁rue ▁de ▁La ▁Ba ume , ▁Paris . ▁As ▁an ▁anti qu arian ▁Ros enberg ▁began ▁bu ying ▁works ▁by ▁Cub ist ▁artists . ▁By ▁ 1 9 1 4 ▁his ▁collection ▁included ▁works ▁by ▁Pablo ▁Pic asso , ▁Jean ▁Met z inger , ▁Albert ▁G le izes , ▁August e ▁Her bin , ▁and ▁Juan ▁Gr is . ▁After ▁serving ▁in ▁World ▁War ▁I ▁( 1 9 1 6 - 1 9 1 7 ) ▁he ▁purs ued ▁his ▁interests ▁and ▁represented ▁these ▁and ▁other ▁artists , ▁and ▁by ▁the ▁end ▁of ▁the ▁war ▁opened ▁a ▁new ▁show ▁space , ▁Galerie ▁de ▁L ' E ff ort ▁Mod erne . ▁ ▁Biography ▁Lé once ▁Ros enberg ▁studied ▁in ▁London ▁and ▁Ant werp ▁visit ing ▁gall eries ▁and ▁museum s ▁in ▁his ▁free ▁time . ▁ ▁After ▁returning ▁to ▁Paris ▁he ▁worked ▁with ▁his ▁brother ▁Paul