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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 Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 N. Fernandez, An order of primeness, F(p) 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 CROSSREFS Sequence in context: A114306 A009188 A138299 * A176656 A248903 A138945 Adjacent sequences:  A050432 A050433 A050434 * A050436 A050437 A050438 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 April 4 20:53 EDT 2020. Contains 333229 sequences. (Running on oeis4.)