sentence1: very commonly , and invariably in papers working with the hdm 34 , 35 , 37 , it has been imposed that no normal electrical current flows in the immediate neighborhood of a surface 15 , 57 .
sentence2: very commonly , and invariably in papers working with the hdm 30 , 31 , 33 , it has been imposed that no normal electrical current flows in the immediate neighborhood of a surface 15 , 53 .