This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A308890 #6 Jul 01 2019 23:03:05 %S A308890 1,1,1,1,2,2,2,3,3,2,2,2,2,3,3,2,3,4,4,3,1,2,4,3,2,1,4,4,4,3,1,2,4,4, %T A308890 2,1,1,1,4,4,1,1,1,3,2,4,4,1,1,1,3,3,4,3,1,1,1,3,2,4,4,2,3,1,2,1,2,2, %U A308890 2,2,1,2,2,3,3,2,4,3,2,1,2,2,2,3,2,2,2,2,1,2,2,3,3,2,1 %N A308890 Follow along the squares in the square spiral (as in A274640); in each square write the smallest positive number that a knight placed at that square cannot see. %C A308890 Similar to A274640, except that here we consider the mex of squares that are a knight's moves rather than queen's moves. %C A308890 Since there are at most 4 earlier cells in the spiral at a knight's move from any square, a(n) <= 5. %C A308890 This is obtained by adding 1 to the terms of A308884. "Mex" here means minimal positive excluded value. %Y A308890 Cf. A247640, A274641, A308885-A308895. %K A308890 nonn %O A308890 1,5 %A A308890 _N. J. A. Sloane_, Jul 01 2019