

A071904


Odd composite numbers.


30



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
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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 (n1)^(n1) == 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


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))*(1A010051(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


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 *)


PROG

(Haskell)
a071904 n = a071904_list !! (n1)
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


CROSSREFS

Cf. A002808, A014076, A002000, A005408, A000035, A010051, A020639, A162022.
Sequence in context: A079364 A160666 A039769 * A014076 A067800 A155474
Adjacent sequences: A071901 A071902 A071903 * A071905 A071906 A071907


KEYWORD

nice,nonn,easy


AUTHOR

Shyam Sunder Gupta, Jun 12 2002


STATUS

approved



