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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. 7
 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 G. C. Greubel, Table of n, a(n) for n = 0..1000 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.] BackIssues.com, Scientific American March 1964 back issue 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: A027201 A026933 A052171 * A030281 A289847 A063767 Adjacent sequences:  A168019 A168020 A168021 * A168023 A168024 A168025 KEYWORD easy,nonn AUTHOR Alonso del Arte, Nov 16 2009 STATUS approved

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