R. Alter, Computations and generalizations on a remark of Ramanujan, pp. 182-196 of "Analytic Number Theory (Philadelphia, 1980)", ed. M. I. Knopp, Lect. Notes Math., Vol. 899, 1981.
EXAMPLE
4=1^1+3^1=2^1+2^1, 50=1^2+7^2=5^2+5^2, 1729=1^3+12^3=9^3+10^3, etc.
