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 A168023 #24 Feb 16 2025 08:33:11
%S A168023 1,61,139,1009,1279,2281,3109,3571,4591,6361,8419,13399,14341,17359,
%T A168023 19531,23029,35251,39901,44839,46549,51871,55579,61381,73849,76039,
%U A168023 102241,110059,135241,153469,156619
%N A168023 Noncomposite numbers in the northern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.
%H A168023 G. C. Greubel, <a href="/A168023/b168023.txt">Table of n, a(n) for n = 1..1000</a>
%H A168023 Alonso del Arte, <a href="http://www.youtube.com/watch?v=dx24qqBc-PY">Ulam spiral</a> (2009). [Note that "East" and "West" in this video match the cover of Scientific American, but "North" and "South" are switched.]
%H A168023 BackIssues.com, <a href="http://backissues.com/issue/Scientific-American-March-1964">Scientific American March 1964 back issue</a>
%H A168023 MathWorld, <a href="https://mathworld.wolfram.com/PrimeSpiral.html">Prime Spiral</a>
%H A168023 Scientific American, <a href="/A168022/a168022.pdf">March 1964 cover</a>
%H A168023 Wikipedia, <a href="https://en.wikipedia.org/wiki/Ulam_spiral#Construction">Ulam Spiral</a>
%F A168023 Positive numbers of the form 4n^2 - 9n + 6 with no more than two divisors.
%t A168023 Select[Table[4 n^2 - 9 n + 6, {n, 200}], Length[Divisors[ # ]] < 3 &]
%Y A168023 Cf. A054556, all numbers of the form 4n^2 - 9n + 6. Noncomposites of the eastern ray are in A168022. Primes of the northeastern ray are in A073337. Noncomposites of the northwestern ray are in A168024. Noncomposites of the western ray are in A168025. Noncomposites of the southwestern ray are in A168026. Noncomposites of the southern ray are in A168027.
%K A168023 easy,nonn
%O A168023 1,2
%A A168023 _Alonso del Arte_, Nov 16 2009