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 A274917 #65 Mar 07 2020 13:50:20 %S A274917 1,2,3,4,2,3,2,4,3,1,4,1,2,5,1,3,1,4,1,4,1,3,1,2,4,2,3,2,3,4,1,3,4,2, %T A274917 4,2,3,5,2,3,2,3,2,4,2,4,3,1,3,1,4,1,4,1,2,3,2,4,2,1,3,1,5,1,2,4,1,4, %U A274917 1,4,1,4,1,3,1,3,1,2,4,2,4,2,3,2,3,2,3,4,1,4,1,3,1,3,4,2,4,2,3,4,1,3,5,2,3 %N A274917 Square spiral in which each new term is the least positive integer distinct from its (already assigned) eight neighbors. %C A274917 The largest element is 5 and it is also the element with lower density in the spiral. %C A274917 See A275609 for proof that 5 is maximal and for further comments. - _N. J. A. Sloane_, Mar 24 2019 %H A274917 F. Michel Dekking, Jeffrey Shallit, and N. J. A. Sloane, <a href="https://www.combinatorics.org/ojs/index.php/eljc/article/view/v27i1p52/8039">Queens in exile: non-attacking queens on infinite chess boards</a>, Electronic J. Combin., 27:1 (2020), #P1.52. %F A274917 a(n) = A275609(n) + 1. - _Omar E. Pol_, Nov 14 2016 %e A274917 Illustration of initial terms as a spiral (n = 0..168): %e A274917 . %e A274917 . 2 - 3 - 2 - 1 - 5 - 1 - 3 - 1 - 2 - 4 - 2 - 4 - 2 %e A274917 . | | %e A274917 . 4 1 - 4 - 3 - 2 - 4 - 2 - 4 - 3 - 1 - 3 - 1 3 %e A274917 . | | | | %e A274917 . 2 3 2 - 1 - 5 - 1 - 3 - 1 - 2 - 4 - 2 4 2 %e A274917 . | | | | | | %e A274917 . 1 5 4 3 - 2 - 4 - 2 - 4 - 3 - 1 3 1 3 %e A274917 . | | | | | | | | %e A274917 . 4 2 1 5 1 - 3 - 1 - 5 - 2 4 2 4 2 %e A274917 . | | | | | | | | | | %e A274917 . 1 3 4 2 4 2 - 4 - 3 1 3 1 3 1 %e A274917 . | | | | | | | | | | | | %e A274917 . 4 2 1 3 1 3 1 - 2 4 2 4 2 4 %e A274917 . | | | | | | | | | | | %e A274917 . 1 3 4 2 4 2 - 4 - 3 - 1 3 1 3 1 %e A274917 . | | | | | | | | | %e A274917 . 4 2 1 3 1 - 3 - 1 - 2 - 4 - 2 4 2 4 %e A274917 . | | | | | | | %e A274917 . 1 3 4 2 - 4 - 2 - 4 - 3 - 1 - 3 - 1 3 1 %e A274917 . | | | | | %e A274917 . 4 2 1 - 3 - 1 - 3 - 1 - 2 - 4 - 2 - 4 - 2 4 %e A274917 . | | | %e A274917 . 1 3 - 4 - 2 - 4 - 2 - 4 - 3 - 1 - 3 - 1 - 3 - 1 %e A274917 . | %e A274917 . 2 - 5 - 1 - 3 - 1 - 3 - 1 - 2 - 4 - 2 - 4 - 2 - 4 %e A274917 . %e A274917 a(13) = 5 is the first "5" in the sequence and its four neighbors are 4 (southwest), 3 (south), 1 (southeast) and 2 (east) when a(13) is placed in the spiral. %e A274917 a(157) = 5 is the 6th "5" in the sequence and it is also the first "5" that is below the NE-SW main diagonal of the spiral (see the second term in the last row of the above diagram). %Y A274917 Cf. A274913, A274921, A275609, A278354 (number of neighbors). %K A274917 nonn %O A274917 0,2 %A A274917 _Omar E. Pol_, Jul 11 2016