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A066246 a(n) = 0 unless n is a composite number A002808(k) then a(n) = k.

%I A066246 #25 Jul 29 2025 08:38:56
%S A066246 0,0,0,1,0,2,0,3,4,5,0,6,0,7,8,9,0,10,0,11,12,13,0,14,15,16,17,18,0,
%T A066246 19,0,20,21,22,23,24,0,25,26,27,0,28,0,29,30,31,0,32,33,34,35,36,0,37,
%U A066246 38,39,40,41,0,42,0,43,44,45,46,47,0,48,49,50,0,51,0,52,53,54,55,56,0,57
%N A066246 a(n) = 0 unless n is a composite number A002808(k) then a(n) = k.
%H A066246 Charles R Greathouse IV, <a href="/A066246/b066246.txt">Table of n, a(n) for n = 1..10000</a>
%F A066246 a(n) = A239968(n) + A010051(n) - 1. - _Reinhard Zumkeller_, Mar 30 2014
%F A066246 a(n) = A065855(n)*A005171(n). - _Ridouane Oudra_, Jul 29 2025
%t A066246 Module[{k=1},Table[If[CompositeQ[n],k;k++,0],{n,100}]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jan 03 2019 *)
%o A066246 (PARI) a(n)=if(isprime(n),0,max(0,n-primepi(n)-1)) \\ _Charles R Greathouse IV_, Aug 21 2011
%o A066246 (Haskell)
%o A066246 import Data.List (unfoldr, genericIndex)
%o A066246 a066246 n = genericIndex a066246_list (n - 1)
%o A066246 a066246_list = unfoldr x (1, 1, a002808_list) where
%o A066246    x (i, z, cs'@(c:cs)) | i == c = Just (z, (i + 1, z + 1, cs))
%o A066246                         | i /= c = Just (0, (i + 1, z, cs'))
%o A066246 -- _Reinhard Zumkeller_, Jan 29 2014
%Y A066246 Cf. A002808, A239968, A010051, A049084.
%Y A066246 Cf. A026233, A026238, A065855, A005171.
%K A066246 nonn
%O A066246 1,6
%A A066246 _Reinhard Zumkeller_, Dec 09 2001