[math-fun] n-complements, squared

Sudipta Das sent me an interesting question. Define the n-complement of a number j as the number k, such that corresponding digits of j and k always add to n. For example, the 9-complement of 73149812 is 26850187. Sudipta noticed: ( 225 , 441 ) are 6-complements, and both are square numbers ( 39866596 , 82355625 ) are 11-complements, and both are square numbers I wondered several things, and I don't have answers. 1. Does this concept of n-complements has a better known name? 2. What is the next set of n-complementary square numbers? 3. What other interesting things can be done with n-complements? --Ed Pegg Jr, www.mathpuzzle.com