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

Author

Alonso del Arte, Nov 16 2009

Keywords

• easy
• nonn

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

Crossrefs

Cf. A054567.

Programs

Mathematica

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

Formula

Positive numbers of the form 4n^2 - 7n + 4 with no more than two divisors.