a(n) = Sum_{y=1..n} Sum_{x=1..n} floor((x^k + y^k)^(1/k)) with k = 2.

Terms

a(0) =1a(1) =7a(2) =23a(3) =54a(4) =103a(5) =175a(6) =276a(7) =409a(8) =579a(9) =791a(10) =1050a(11) =1360a(12) =1724a(13) =2149a(14) =2640a(15) =3198a(16) =3832a(17) =4543a(18) =5337a(19) =6217a(20) =7192a(21) =8265a(22) =9437a(23) =10716a(24) =12103a(25) =13609a(26) =15231a(27) =16978a(28) =18857a(29) =20869