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A050435
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a(n) = composite(composite(n)), where composite = A002808, composite numbers.
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12
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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
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OFFSET
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1,1
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COMMENTS
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Second-order composite numbers.
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LINKS
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FORMULA
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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
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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
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Select[ Range[ 6, 150 ], ! PrimeQ[ # ] && ! PrimeQ[ # - PrimePi[ # ] - 1 ] & ]
With[{cmps=Select[Range[200], CompositeQ]}, Table[cmps[[cmps[[n]]]], {n, 70}]] (* Harvey P. Dale, Feb 18 2018 *)
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PROG
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(Haskell)
a050435 = a002808 . a002808
a050435_list = map a002808 a002808_list
(PARI) composite(n)=my(k=-1); while(-n + n += -k + k=primepi(n), ); n \\ M. F. Hasler
(Python)
from sympy import composite
def a(n): return composite(composite(n))
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CROSSREFS
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KEYWORD
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easy,nonn,nice
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AUTHOR
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Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999
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EXTENSIONS
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STATUS
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approved
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