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A168025 Noncomposite numbers in the western ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American. 7
1, 19, 151, 1621, 2731, 3631, 4129, 7789, 11719, 12601, 14461, 15439, 17491, 20809, 28309, 29671, 32491, 41719, 59659, 69829, 78541, 83089, 85411, 92569, 97501, 115771, 132679, 138571 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

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 - 7n + 4 with no more than two divisors.

MATHEMATICA

Select[Table[4 n^2 - 7 n + 4, {n, 0, 199}], Length[Divisors[ # ]] < 3 &]

CROSSREFS

Cf. A054567.

Sequence in context: A066668 A111884 A126514 * A160431 A010825 A022711

Adjacent sequences:  A168022 A168023 A168024 * A168026 A168027 A168028

KEYWORD

easy,nonn

AUTHOR

Alonso del Arte, Nov 16 2009

STATUS

approved

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Last modified December 11 12:33 EST 2019. Contains 329916 sequences. (Running on oeis4.)