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A168022 Noncomposite numbers in the eastern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American. 7
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)
OFFSET

0,2

COMMENTS

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.

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 - 3n + 1 with no more than two divisors.

MATHEMATICA

Select[Table[4 n^2 - 3 n + 1, {n, 0, 199}], Length[Divisors[ # ]] < 3 &]

CROSSREFS

Cf. A054552, all numbers of the form 4n^2 - 3n + 1. Primes of northeastern ray are in A073337. Noncomposites of the northern ray are in A168023. 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: A027201 A026933 A052171 * A030281 A289847 A063767

Adjacent sequences:  A168019 A168020 A168021 * A168023 A168024 A168025

KEYWORD

easy,nonn

AUTHOR

Alonso del Arte, Nov 16 2009

STATUS

approved

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Last modified November 20 08:12 EST 2017. Contains 294962 sequences.