cp's OEIS Frontend

A168024 Noncomposite numbers in the northwestern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.

%I A168024 #19 Feb 16 2025 08:33:11
%S A168024 1,5,17,37,101,197,257,401,577,677,1297,1601,2917,3137,4357,5477,7057,
%T A168024 8101,8837,12101,13457,14401,15377,15877,16901,17957,21317,22501,
%U A168024 24337,25601,28901,30977,32401,33857
%N A168024 Noncomposite numbers in the northwestern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.
%H A168024 G. C. Greubel, <a href="/A168024/b168024.txt">Table of n, a(n) for n = 0..1000</a>
%H A168024 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 A168024 BackIssues.com, <a href="http://backissues.com/issue/Scientific-American-March-1964">Scientific American March 1964 back issue</a>
%H A168024 MathWorld, <a href="https://mathworld.wolfram.com/PrimeSpiral.html">Prime Spiral</a>
%H A168024 Scientific American, <a href="/A168022/a168022.pdf">March 1964 cover</a>
%H A168024 Wikipedia, <a href="https://en.wikipedia.org/wiki/Ulam_spiral#Construction">Ulam Spiral</a>
%F A168024 Positive numbers of the form 4n^2 + 1 with no more than two divisors.
%t A168024 Select[Table[4 n^2 + 1, {n, 0, 99}], Length[Divisors[ # ]] < 3 &]
%Y A168024 Essentially the same sequence as A002496, A121326, A163588.
%Y A168024 Cf. A053755, all numbers of the form 4n^2 + 1. Noncomposites of the eastern ray are in A168022. Primes of the northeastern ray are in A073337. Noncomposites of the northern ray are in A168023. Primes of the northwestern ray are in A121326 (the same as this sequence but without the initial 1). 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 A168024 easy,nonn
%O A168024 0,2
%A A168024 _Alonso del Arte_, Nov 16 2009