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Search: id:A002808
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| A002808 |
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The composite numbers: numbers n of the form x*y for x > 1 and y > 1.
(Formerly M3272 N1322)
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+0
474
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| 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 84, 85, 86, 87, 88
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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The natural numbers 1,2,... are divided into three sets: 1 (the unit), the primes (A000040) and the composite numbers (A002808).
The number of composite numbers <= n (A065855) = n - pi(n) (A000720) - 1.
m is composite iff sigma(m)+phi(m)>2m. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 27 2005
The composite numbers have the semiprimes A001358 as primitive elements.
Numbers that have more than one prime factor. - Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Sep 01 2009
d(n)>2 [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 17 2009]
Smallest nonprime>nth nonprime. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 29 2009]
Number of prime divisors of n (counted with multiplicity)>1. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 30 2009]
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REFERENCES
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T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 2.
A. E. Bojarincev, Asymptotic expressions for the n-th composite number, Univ. Mat. Zap. 6:21-43 (1967). - In Russian.
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers. 3rd ed., Oxford Univ. Press, 1954, p. 2.
L. Panaitopol, Some Properties of the Series of Composed Numbers, J. Inequalities in Pure and Applied Mathematics. 2(2): Article 38, 2000.
Clifford A. Pickover, A Passion for Mathematics, Wiley, 2005; see p. 51.
J. B. Rosser and L. Schoenfeld, Approximate formulas for some functions of prime numbers, Illinois J. Math. 6: 64-94 (1962).
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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N. J. A. Sloane, Table of n, a(n) for n = 1..17737 [composites up to 20000]
C. K. Caldwell, Composite Numbers
Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.
J. Inequalities in Pure and Applied Mathematics.
Index entries for "core" sequences
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MAPLE
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t := []: for n from 2 to 20000 do if isprime(n) then else t := [op(t), n]; fi; od: t;
remove(isprime, [$3..89]); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Mar 19 2007
A002808 := proc(n) local n ; if n = 1 then 4; else for a from procname(n-1)+1 do if not isprime(a) then return a; end if; end do ; end if; end proc; [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Oct 27 2009]
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MATHEMATICA
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Composite[n_Integer] := FixedPoint[n + PrimePi[ # ] + 1 &, n + PrimePi[n] + 1]; Array[Composite, 71] (from Robert G. Wilson v (rgwv(AT)rgwv.com), Jan 13 2006)
Select[Range[100], ! PrimeQ[ # ] && ! (# == 1) && ! (# == 0) &] - Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Mar 30 2006 [Corrected by Barbarel Tres Mil, Feb 28 2009]
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PROGRAM
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(PARI) A002808(n)={for(k=0, primepi(n), isprime(n++)&k--); n} [From M. F. Hasler (MHasler(AT)univ-ag.fr), Oct 31 2008]
(PARI) A002808(n)={ my(k=-1); while( -n + n += -k + k=primepi(n), ); n} [From M. F. Hasler (mhasler(AT)univ-ag.fr), Nov 11 2009]
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CROSSREFS
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Cf. A000040, A018252, A008578, A065090.
a(n) = A136527(n, n).
Cf. A000005, A001222. [From Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Oct 30 2009]
Sequence in context: A167376 A133576 A088224 this_sequence A018252 A141468 A077091
Adjacent sequences: A002805 A002806 A002807 this_sequence A002809 A002810 A002811
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KEYWORD
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nonn,nice,easy,core
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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