A168025 Noncomposite numbers in the western ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.

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

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