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A050435 a(n) = composite(composite(n)), where composite = A002808, composite numbers. 12
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; text; internal format)
OFFSET
1,1
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. - Jaroslav Krizek, Mar 14 2010
LINKS
N. Fernandez, An order of primeness [cached copy, included with permission of the author]
FORMULA
Let C(n) be the n-th composite number, with C(1)=4. Then these are numbers C(C(n)).
a(n) = n + 2n/log n + O(n/log^2 n). - Charles R Greathouse IV, Jun 25 2017
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.
MATHEMATICA
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 *)
PROG
(Haskell)
a050435 = a002808 . a002808
a050435_list = map a002808 a002808_list
-- Reinhard Zumkeller, Jan 12 2013
(PARI) composite(n)=my(k=-1); while(-n + n += -k + k=primepi(n), ); n \\ M. F. Hasler
a(n)=composite(composite(n)) \\ Charles R Greathouse IV, Jun 25 2017
(Python)
from sympy import composite
def a(n): return composite(composite(n))
print([a(n) for n in range(1, 65)]) # Michael S. Branicky, Sep 12 2021
CROSSREFS
Sequence in context: A114306 A009188 A138299 * A176656 A369002 A369976
KEYWORD
easy,nonn,nice
AUTHOR
Michael Lugo (mlugo(AT)thelabelguy.com), Dec 22 1999
EXTENSIONS
More terms from Robert G. Wilson v, Dec 20 2000
STATUS
approved

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Last modified March 28 20:05 EDT 2024. Contains 371254 sequences. (Running on oeis4.)