A168023 - OEIS

%I #24 Feb 16 2025 08:33:11

%S 1,61,139,1009,1279,2281,3109,3571,4591,6361,8419,13399,14341,17359,

%T 19531,23029,35251,39901,44839,46549,51871,55579,61381,73849,76039,

%U 102241,110059,135241,153469,156619

%N Noncomposite numbers in the northern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.

%H G. C. Greubel, <a href="/A168023/b168023.txt">Table of n, a(n) for n = 1..1000</a>

%H 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 BackIssues.com, <a href="http://backissues.com/issue/Scientific-American-March-1964">Scientific American March 1964 back issue</a>

%H MathWorld, <a href="https://mathworld.wolfram.com/PrimeSpiral.html">Prime Spiral</a>

%H Scientific American, <a href="/A168022/a168022.pdf">March 1964 cover</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Ulam_spiral#Construction">Ulam Spiral</a>

%F Positive numbers of the form 4n^2 - 9n + 6 with no more than two divisors.

%t Select[Table[4 n^2 - 9 n + 6, {n, 200}], Length[Divisors[ # ]] < 3 &]

%Y 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 easy,nonn,changed

%O 1,2

%A _Alonso del Arte_, Nov 16 2009