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 A357376 #37 Nov 06 2022 09:05:24 %S A357376 1,26,1016,5136,39639,203100,2729736,32264250,42119062,1065799391, %T A357376 12444190246 %N A357376 The lowest number on Ulam Spiral for which all numbers in the square which is centered at a(n) and spans n-1 spaces in each cardinal direction are nonprime. %C A357376 The n-th term creates a (2*n-1) X (2*n-1) square centered at a(n). %H A357376 Samuel Harkness, <a href="/A357376/a357376.jpg">Illustration of terms 1 through 4</a> %H A357376 Rémy Sigrist, <a href="/A357376/a357376.txt">C++ program</a> %e A357376 For n=2, test squares with 1 layer of numbers around the center number until a square is enclosed with only nonprimes. %e A357376 . %e A357376 65 64 63 62 61 60 59 58 57 %e A357376 . %e A357376 66 37 36 35 34 33 32 31 56 %e A357376 . %e A357376 67 38 17 16 15 14 13 30 55 %e A357376 . %e A357376 68 39 18 5 4 3 12 29 54 %e A357376 . %e A357376 69 40 19 6 1 2 11 28 53 %e A357376 . +--------------+ %e A357376 70 41 20 7 8 9 | 10 27 52 | %e A357376 . | | %e A357376 71 42 21 22 23 24 | 25 26 51 | %e A357376 . | | %e A357376 72 43 44 45 46 47 | 48 49 50 | %e A357376 . +--------------+ %e A357376 73 74 75 76 77 78 79 80 81 %e A357376 . %e A357376 The first number tested, 1, does not work because 2, 3, 5 and 7 are in the 1-layered square and are prime. The first square that works is 26 because 10, 27, 52, 25, 26, 51, 48, 49, and 50 are all nonprime. %o A357376 (C++) See Links section. %Y A357376 Cf. A000040, A115258, A136626, A225073. %K A357376 nonn,more %O A357376 1,2 %A A357376 _Samuel Harkness_, Sep 26 2022 %E A357376 a(8)-a(11) from _Rémy Sigrist_, Sep 27 2022