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 A047845 a(n) = (m-1)/2, where m is the n-th odd nonprime (A014076(n)). 31
 0, 4, 7, 10, 12, 13, 16, 17, 19, 22, 24, 25, 27, 28, 31, 32, 34, 37, 38, 40, 42, 43, 45, 46, 47, 49, 52, 55, 57, 58, 59, 60, 61, 62, 64, 66, 67, 70, 71, 72, 73, 76, 77, 79, 80, 82, 84, 85, 87, 88, 91, 92, 93, 94, 97, 100, 101, 102, 103, 104, 106, 107, 108, 109, 110, 112, 115 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also (starting with 2nd term) numbers of the form 2xy+x+y for x and y positive integers. This is also the numbers of sticks needed to construct a two-dimensional rectangular lattice of unit squares. See A090767 for the three-dimensional generalization. - John H. Mason, Feb 02 2004 Note that if k is not in this sequence, then 2*k+1 is prime. - Jose Brox (tautocrona(AT)terra.es), Dec 29 2005 Values of k for which A073610(2k+3)=0; values of k for which A061358(2k+3)=0. - Graeme McRae, Jul 18 2006 This sequence also arises in the following way: take the product of initial odd numbers, i.e., the product (2n+1)!/(n!*2^n) and factor it into prime numbers. The result will be of the form 3^f(3)*5^f(5)*7^f(7)*11^f(11)... . Then f(3)/f(5) = 2, f(3)/f(7) = 3, f(3)/f(11) = 5, ... and this sequence forms (for sufficiently large n, of course) the sequence of natural numbers without 4,7,10,12,..., i.e., these numbers are what is lacking in the present sequence. - Andrzej Staruszkiewicz (uszkiewicz(AT)poczta.onet.pl), Nov 10 2007 Also "flag short numbers", i.e., number of dots that can be arranged in successive rows of K, K+1, K, K+1, K, ..., K+1, K (assuming there is a total of L > 1 rows of size K > 0). Adapting Skip Garibaldi's terms, sequence A053726 would be "flag long numbers" because those patterns begin and end with the long lines. If you convert dots to sticks, you get the lattice that John H. Mason mentioned. - Juhani Heino, Oct 11 2014 Numbers k such that (2*k)!/(2*k + 1) is an integer. - Peter Bala, Jan 24 2017 Except for a(1)=0: numbers of the form k == j (mod 2j+1), j >= 1, k > 2j+1. - Bob Selcoe, Nov 07 2017 LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Slate, Article about USA flag patterns -- this is where Skip Garibaldi gave definitions. FORMULA A193773(a(n)) > 1 for n > 1. - Reinhard Zumkeller, Jan 02 2013 MAPLE for n from 0 to 120 do if irem(factorial(2*n), 2*n+1) = 0 then print(n); end if; end do: # Peter Bala, Jan 24 2017 MATHEMATICA (Select[Range[1, 231, 2], PrimeOmega[#] != 1 &] - 1)/2 (* Jayanta Basu, Aug 11 2013 *) PROG (Haskell) a047845 = (`div` 2) . a014076 -- Reinhard Zumkeller, Jan 02 2013 (Magma) [(n-1)/2 : n in [1..350] | (n mod 2) eq 1 and not IsPrime(n)]; // G. C. Greubel, Oct 16 2023 (SageMath) [(n-1)/2 for n in (1..350) if n%2==1 and not is_prime(n)] # G. C. Greubel, Oct 16 2023 (PARI) print1(0, ", "); forcomposite(n=1, 250, if(1==n%2, print1((n-1)/2, ", "))); \\ Joerg Arndt, Oct 16 2023 CROSSREFS Complement of A005097. Sequence in context: A025357 A222949 A144020 * A097703 A310674 A104036 Adjacent sequences: A047842 A047843 A047844 * A047846 A047847 A047848 KEYWORD easy,nonn AUTHOR Enoch Haga EXTENSIONS Name edited by Jon E. Schoenfield, Oct 16 2023 STATUS approved

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Last modified June 19 06:59 EDT 2024. Contains 373492 sequences. (Running on oeis4.)