a(n) = smallest k with concat(1,k) and concat(n,k) both square numbers.

Terms

a(0) =225a(1) =6a(2) =1025a(3) =6a(4) =225a(6) =80625a(7) =225a(8) =19025a(10) =5625a(12) =96a(13) =69a(14) =44a(15) =21a(18) =225a(19) =6a(20) =8734765625a(22) =5625a(23) =863225a(24) =80625a(25) =6a(26) =40625a(27) =225a(28) =890625