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A055399
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Stages of sieve of Eratosthenes needed to identify n as prime or composite.
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7
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1, 1, 2, 1, 2, 1, 2, 1, 3, 1, 3, 1, 2, 1, 3, 1, 3, 1, 2, 1, 3, 1, 3, 1, 2, 1, 4, 1, 4, 1, 2, 1, 3, 1, 4, 1, 2, 1, 4, 1, 4, 1, 2, 1, 4, 1, 4, 1, 2, 1, 5, 1, 3, 1, 2, 1, 5, 1, 5, 1, 2, 1, 3, 1, 5, 1, 2, 1, 5, 1, 5, 1, 2, 1, 4, 1, 5, 1, 2, 1, 5, 1, 3, 1, 2, 1, 5
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OFFSET
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3,3
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LINKS
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T. D. Noe, Table of n, a(n) for n = 3..10000
H. B. Meyer, Eratosthenes' sieve
J. Britton, Sieve of Eratosthenes Applet
C. K. Caldwell, The Prime Glossary, sieve of Eratosthenes
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FORMULA
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If n is composite, a(n) = A055396(n); if n is prime, a(n) = A000040(firstprimeabove(sqrt(n)).
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EXAMPLE
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a(7)=2 because 7 is not removed by the first two stages of the sieve, but is less than the square of the second prime (though not the square of the first); a(35)=3 because 35 is removed in the third stage as a multiple of 5.
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MATHEMATICA
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a[n_ /; !PrimeQ[n]] := PrimePi[ FactorInteger[n][[1, 1]]]; a[n_ /; PrimeQ[n]] := PrimePi[ NextPrime[ Sqrt[n]]]; Table[a[n], {n, 3, 107}](* Jean-François Alcover, Jun 11 2012, after formula *)
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CROSSREFS
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Cf. A000040, A002808, A004280, A038179, A055396, A055397, A055397.
Cf. A054403; A055398; A083269.
Sequence in context: A033103 A060775 A175494 * A029426 A085342 A025825
Adjacent sequences: A055396 A055397 A055398 * A055400 A055401 A055402
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KEYWORD
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nice,nonn
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AUTHOR
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Henry Bottomley, May 15 2000
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STATUS
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approved
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