This was conjectured by Oppenheim in 1953 and proved by Birch, Davenport, and Ridout for n at least 21, and by Davenport and Heilbronn for diagonal forms in five variables.
Oppenheim proposed this theory in 1953, which was subsequently verified by Birch, Davenport, and Ridout for n at least 21, and by Davenport and Heilbronn for diagonal forms involving five variables.