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A168023 Noncomposite numbers in the northern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American. 6
1, 61, 139, 1009, 1279, 2281, 3109, 3571, 4591, 6361, 8419, 13399, 14341, 17359, 19531, 23029, 35251, 39901, 44839, 46549, 51871, 55579, 61381, 73849, 76039, 102241, 110059, 135241, 153469, 156619 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..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 - 9n + 6 with no more than two divisors.

MATHEMATICA

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

CROSSREFS

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.

Sequence in context: A063337 A306750 A142172 * A044312 A044693 A185662

Adjacent sequences:  A168020 A168021 A168022 * A168024 A168025 A168026

KEYWORD

easy,nonn

AUTHOR

Alonso del Arte, Nov 16 2009

STATUS

approved

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Last modified December 11 18:34 EST 2019. Contains 329925 sequences. (Running on oeis4.)