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A071904 Odd composite numbers. 24
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 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; A000035(a(n))*(1-A010051(a(n))) = 1; A020639(a(n)) = A162022(n). [Reinhard Zumkeller, Oct 10 2011]

Composite numbers n such that (n-1)^(n-1) == 1 (mod n). [Michel Lagneau, Feb 18 2012]

LINKS

Zak Seidov and K. D. Bajpai, Table of n, a(n) for n = 1..10000 (first 1000 terms from Zak Seidov)

FORMULA

a(n) ~ 2n. - Charles R Greathouse IV, Jul 02 2013

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).

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 !! (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

CROSSREFS

Cf. A002808.

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

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Last modified November 29 01:24 EST 2014. Contains 250479 sequences.