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A168022
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Non-composite numbers in the eastern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.
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4
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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)
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OFFSET
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0,2
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COMMENTS
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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.
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LINKS
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Table of n, a(n) for n=0..37.
YouTube, Ulam spiral. 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
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FORMULA
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Positive numbers of the form 4n^2 - 3n + 1 with no more than two divisors.
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MATHEMATICA
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Select[Table[4 n^2 - 3 n + 1, {n, 0, 199}], Length[Divisors[ # ]] < 3 &]
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CROSSREFS
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Cf. A054552, all numbers of the form 4n^2 - 3n + 1. Primes of northeastern ray are in A073337. Non-composites of northern ray are in A168023. Non-composites of northwestern ray are in A168024. Non-composites of western ray are in A168025. Non-composites of southwestern ray are in A168026. Non-composites of southern ray are in A168027.
Sequence in context: A027201 A026933 A052171 * A030281 A063767 A161559
Adjacent sequences: A168019 A168020 A168021 * A168023 A168024 A168025
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KEYWORD
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easy,nonn
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AUTHOR
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Alonso del Arte, Nov 16 2009
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STATUS
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approved
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