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A168022 Noncomposite numbers in the eastern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American. 8
1, 2, 11, 53, 127, 233, 541, 743, 977, 1871, 3511, 4001, 4523, 5077, 9851, 11503, 12377, 14221, 16193, 19391, 20521, 21683, 22877, 24103, 29327, 30713, 33581, 42953, 55343, 57241, 63127, 67211, 80231, 84827, 91961, 101921, 104491, 123377 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
COMMENTS
Although 1 was not considered a prime number in Ulam's time, the March 1964 cover of Scientific American shows 1 highlighted in the same way as the primes.
LINKS
Alonso del Arte, Ulam spiral (2009). [Note that "East" and "West" in this video match the cover of Scientific American, but "North" and "South" are switched.]
MathWorld, Prime Spiral
Scientific American, March 1964 cover
Wikipedia, Ulam Spiral
FORMULA
Positive numbers of the form 4n^2 - 3n + 1 with no more than two divisors.
MATHEMATICA
Select[Table[4 n^2 - 3 n + 1, {n, 0, 199}], Length[Divisors[ # ]] < 3 &]
CROSSREFS
Cf. A054552, all numbers of the form 4n^2 - 3n + 1. Primes of northeastern ray are in A073337. Noncomposites of the northern ray are in A168023. 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.
Sequence in context: A346925 A052171 A333542 * A355974 A030281 A289847
KEYWORD
easy,nonn
AUTHOR
Alonso del Arte, Nov 16 2009
STATUS
approved

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Last modified March 28 20:05 EDT 2024. Contains 371254 sequences. (Running on oeis4.)