▁career ▁as ▁professor ▁in ▁La ▁Pl ata . ▁ ▁During ▁Ein stein ' s ▁visit ▁to ▁Argentina ▁in ▁ 1 9 2 5 ▁they ▁had ▁a ▁conversation ▁about ▁the ▁differential ▁equation ▁of ▁a ▁point - source ▁gravit ational ▁field , ▁which ▁resulted ▁in ▁a ▁paper ▁published ▁by ▁Lo ed el ▁in ▁Phys ikal ische ▁Zeitschrift . ▁It ▁is ▁claimed ▁that ▁this ▁is ▁the ▁first ▁research ▁paper ▁on ▁relativ ity ▁ever ▁published ▁by ▁a ▁Latin ▁American ▁scient ist . ▁ ▁Lo ed el ▁Pal um bo ▁then ▁spent ▁some ▁time ▁in ▁Germany ▁working ▁with ▁Er win ▁Sch ro ed inger ▁and ▁Max ▁Plan ck . ▁He ▁returned ▁to ▁Argentina ▁in ▁ 1 9 3 0 ▁and ▁from ▁there ▁on ▁concentr ated ▁on ▁teaching . ▁He ▁published ▁several ▁scientific ▁papers ▁during ▁his ▁career ▁in ▁international ▁journ als ▁and ▁wrote ▁several ▁books ▁( in ▁Spanish ). ▁ ▁Lo ed el ▁diagram ▁ ▁Max ▁Born ▁( 1 9 2 0 ) ▁and ▁system atically ▁Paul ▁Gr un er ▁( 1 9 2 1 ) ▁introduced ▁symmetric ▁M ink owski ▁diag rams ▁in ▁German ▁and ▁French ▁papers , ▁where ▁the ▁c t '- axis ▁is ▁per pend icular ▁to ▁the ▁x - axis , ▁as ▁well ▁as ▁the ▁c t - axis ▁per pend icular ▁to ▁the ▁x '- axis ▁( for ▁sources ▁and ▁historical ▁details , ▁see ▁Lo ed el ▁diagram ). ▁ ▁In ▁ 1 9 4 8 ▁and ▁in ▁subsequent ▁papers , ▁Lo ed el ▁independently ▁redis cover ed ▁such ▁diag rams . ▁They ▁were