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 A071904 Odd composite numbers. 52
 9, 15, 21, 25, 27, 33, 35, 39, 45, 49, 51, 55, 57, 63, 65, 69, 75, 77, 81, 85, 87, 91, 93, 95, 99, 105, 111, 115, 117, 119, 121, 123, 125, 129, 133, 135, 141, 143, 145, 147, 153, 155, 159, 161, 165, 169, 171, 175, 177, 183, 185, 187, 189, 195 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Same as A014076 except for the initial term A014076(1)=1 (which is not a composite number). Values of quadratic form (2x + 3)*(2y + 3) = 4xy + 6x + 6y + 9 for x, y >= 0. - Anton Joha, Jan 21 2001 Intersection of A002808 and A005408. - Reinhard Zumkeller, Oct 10 2011 Composite numbers n such that (n-1)^(n-1) == 1 (mod n). - Michel Lagneau, Feb 18 2012 There is a rectangular array of n dots (with both sides > 1) with a unique center point if and only if n is in this sequence. - Peter Woodward, Apr 21 2015 First differences <= 6. Cf. A164510. - Zak Seidov, Sep 22 2016 Let r(n) = (a(n)-1)/(a(n)+1)) if a(n) mod 4 = 1, (a(n)+1)/(a(n)-1)) otherwise; then Product_{n>=1} r(n) = (4/5) * (8/7) * (10/11) * (12/13) * (14/13) * ... = Pi/4. - Dimitris Valianatos, May 24 2017 LINKS Zak Seidov and K. D. Bajpai, Table of n, a(n) for n = 1..10000 (first 1000 terms from Zak Seidov) FORMULA A000035(a(n))*(1-A010051(a(n))) = 1; A020639(a(n)) = A162022(n). - Reinhard Zumkeller, Oct 10 2011 a(n) ~ 2n. - Charles R Greathouse IV, Jul 02 2013 More precisely, a(n) = 2n(1 + 2(1+o(1))/log(n)). - Vladimir Shevelev, Jan 07 2015 EXAMPLE 45 is in the sequence because it is odd and composite (45 = 3 * 3 * 5). 195 is in the sequence because it is odd and composite (195 = 3 * 5 * 13). MAPLE remove(isprime, [seq(2*i+1, i = 1 .. 1000)]); # Robert Israel, Apr 22 2015 # alternative A071904 := proc(n)     if n = 1 then         9;     else         for a from procname(n-1)+2 by 2 do             if not isprime(a) then                 return a;             end if;         end do:     end if; end proc: # R. J. Mathar, Sep 09 2015 MATHEMATICA Select[Table[n, {n, 9, 300, 2}], !PrimeQ[#] &] (* Vladimir Joseph Stephan Orlovsky, Apr 16 2011 *) With[{upto = 200}, Complement[Range[9, upto, 2], Prime[Range[ PrimePi[ upto]]]]] (* Harvey P. Dale, Jan 24 2013 *) With[{upto = 200}, oddsequence=Table[2n+1, {n, 1, upto}]; oddcomposites=Union[Flatten[Range[oddsequenceË†2, upto, 2*oddsequence]]]] (* Ben Engelen, Feb 24 2016 *) PROG (Haskell) a071904 n = a071904_list !! (n-1) a071904_list = filter odd a002808_list -- Reinhard Zumkeller, Oct 10 2011 (PARI) is(n)=n%2 && !isprime(n) && n > 1 \\ Charles R Greathouse IV, Nov 24 2012 (PARI) lista(nn) = forcomposite(n=1, nn, if (n%2, print1(n, ", "))); \\ Michel Marcus, Sep 24 2016 CROSSREFS Cf. A002808, A014076, A002000, A005408, A000035, A010051, A020639, A162022. Cf. A164510. Sequence in context: A160666 A039769 A270574 * A326586 A014076 A067800 Adjacent sequences:  A071901 A071902 A071903 * A071905 A071906 A071907 KEYWORD nice,nonn,easy AUTHOR Shyam Sunder Gupta, Jun 12 2002 STATUS approved

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Last modified May 26 07:53 EDT 2020. Contains 334620 sequences. (Running on oeis4.)