a(n) = Sum_{y=1..n} Sum_{x=1..n} floor((x^k + y^k)^(1/k)) with k = 3.

Terms

a(0) =1a(1) =7a(2) =22a(3) =51a(4) =97a(5) =164a(6) =258a(7) =382a(8) =541a(9) =741a(10) =982a(11) =1271a(12) =1611a(13) =2008a(14) =2466a(15) =2986a(16) =3577a(17) =4241a(18) =4982a(19) =5807a(20) =6715a(21) =7714a(22) =8808a(23) =10000a(24) =11297a(25) =12701a(26) =14217a(27) =15848a(28) =17600a(29) =19477