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A139118
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Numbers with a nonprime number of divisors.
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4
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1, 6, 8, 10, 12, 14, 15, 18, 20, 21, 22, 24, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 100, 102
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OFFSET
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1,2
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COMMENTS
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A000005(a(n)) is nonprime. Complement of A009087. Also, nonprime numbers with nonprime number of divisors.
The sequence consists of those n such that n is not a prime power, or n of the form p^k where k+1 is composite. - Franklin T. Adams-Watters, Apr 09 2009
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LINKS
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EXAMPLE
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15 is in the sequence because it has 4 divisors: 1, 3, 5, and 15. - Emeric Deutsch, Jun 27 2009
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MAPLE
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with(numtheory): a := proc (n) if isprime(tau(n)) = false then n else end if end proc: seq(a(n), n = 1 .. 120); # Emeric Deutsch, Jun 27 2009
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MATHEMATICA
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Select[Range[102], ! PrimeQ[DivisorSigma[0, #]] &] (* Amiram Eldar, Nov 27 2020 *)
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PROG
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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