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A050435
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a(n) = composite(composite(n)), where composite = A002808, composite numbers.
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10
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9, 12, 15, 16, 18, 21, 24, 25, 26, 28, 32, 33, 34, 36, 38, 39, 40, 42, 45, 48, 49, 50, 51, 52, 55, 56, 57, 60, 63, 64, 65, 68, 69, 70, 72, 74, 76, 77, 78, 80, 81, 84, 86, 87, 88, 90, 91, 93, 94, 95, 98, 100, 102, 104, 105, 106, 110, 111, 112, 115, 116, 117, 118, 119
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Second-order composite numbers.
Composites (A002808) with composite (A002808) subscripts. a(n) U A022449(n) = A002808(n). Subsequence of A175251 (composites (A002808) with nonprime (A018252) subscripts), a(n) = A175251(n+1) for n >= 1. [From Jaroslav Krizek (jaroslav.krizek(AT)atlas.cz), Mar 14 2010]
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LINKS
| N. Fernandez, An order of primeness, F(p)
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FORMULA
| Let C(n) be the n-th composite number, with C(1)=4. Then these are numbers C(C(n)).
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EXAMPLE
| The 2nd composite number is 6 and the 6th composite number is 12, so a(2) = 12. a(100) = A002808(A002808(100)) = A002808(133) = 174.
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MATHEMATICA
| Select[ Range[ 6, 150 ], ! PrimeQ[ # ] && ! PrimeQ[ # - PrimePi[ # ] - 1 ] & ]
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CROSSREFS
| Sequence in context: A114306 A009188 A138299 * A176656 A138945 A119486
Adjacent sequences: A050432 A050433 A050434 * A050436 A050437 A050438
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KEYWORD
| easy,nonn,nice
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AUTHOR
| Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999
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EXTENSIONS
| More terms from Robert G. Wilson v (rgwv(AT)rgwv.com), Dec 20 2000
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