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A060687
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Numbers n such that there exist exactly 2 Abelian groups of order n, i.e. A000688(n) = 2.
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9
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4, 9, 12, 18, 20, 25, 28, 44, 45, 49, 50, 52, 60, 63, 68, 75, 76, 84, 90, 92, 98, 99, 116, 117, 121, 124, 126, 132, 140, 147, 148, 150, 153, 156, 164, 169, 171, 172, 175, 188, 198, 204, 207, 212, 220, 228, 234, 236, 242, 244, 245, 260, 261, 268, 275, 276, 279
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| n belongs to this sequence iff exactly one prime in its factorization into prime powers has exponent 2 and all the other primes in the factorization have exponent 1, for example 60 = 2^2 * 3 * 5.
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FORMULA
| n such that A001222(n)-A001221(n) = 1
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MATHEMATICA
| Select[Range[300], PrimeOmega[#]-PrimeNu[#]==1&] (* From Harvey P. Dale, Sep 08 2011 *)
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PROG
| (PARI) for(n=1, 279, if(bigomega(n)-omega(n)==1, print1(n, ", ")))
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CROSSREFS
| Cf. A000688.
Sequence in context: A081619 A038109 A067259 * A181023 A084789 A157650
Adjacent sequences: A060684 A060685 A060686 * A060688 A060689 A060690
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KEYWORD
| nonn
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AUTHOR
| Ahmed Fares (ahmedfares(AT)my-deja.com), Apr 19 2001
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EXTENSIONS
| Corrected and extended by Vladeta Jovovic (vladeta(AT)eunet.rs), Jul 05 2001
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