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 A005097 (Odd primes - 1)/2. 129
 1, 2, 3, 5, 6, 8, 9, 11, 14, 15, 18, 20, 21, 23, 26, 29, 30, 33, 35, 36, 39, 41, 44, 48, 50, 51, 53, 54, 56, 63, 65, 68, 69, 74, 75, 78, 81, 83, 86, 89, 90, 95, 96, 98, 99, 105, 111, 113, 114, 116, 119, 120, 125, 128, 131, 134, 135, 138, 140, 141, 146, 153, 155, 156 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Or, numbers n such that 2n+1 is prime. Also numbers not of the form 2xy+x+y. - Jose Brox (tautocrona(AT)terra.es), Dec 29 2005 This sequence arises if you factor the product of a large number of the first odd numbers into the form 3^n(3)5^n(5)7^n(7)11^n(11)... Then n(3)/n(5) = 2, n(3)/n(7) = 3, n(3)/n(11) = 5, ... . - Andrzej Staruszkiewicz (astar(AT)th.if.uj.edu.pl), May 31 2007 Kohen shows: A king invites n couples to sit around a round table with 2n+1 seats. For each couple, the king decides a prescribed distance d between 1 and n which the two spouses have to be seated from each other (distance d means that they are separated by exactly d-1 chairs). We will show that there is a solution for every choice of the distances if and only if 2n+1 is a prime number [i.e., iff n is in A005097], using a theorem known as Combinatorial Nullstellensatz. - Jonathan Vos Post, Jun 14 2010 Starting from 6, positions at which new primes are seen for Goldbach partitions. E.g., 31 is first seen at 34 from 31+3, so position = 1 + (34-6)/2 = 15. - Bill McEachen, Jul 05 2010 Perfect error-correcting Lee codes of word length n over Z: it is conjectured that these always exist when 2n+1 is a prime, as mentioned in Horak. - Jonathan Vos Post, Sep 19 2011 Also solutions to:  A000010(2*n+1) = n * A000005(2*n+1). - Enrique Pérez Herrero, Jun 07 2012 Solutions of the equation (2*n+1)'=1, where n' is the arithmetic derivative of n. - Paolo P. Lava, Nov 15 2012 A193773(a(n)) = 1. - Reinhard Zumkeller, Jan 02 2013 I conjecture that the set of pairwise sums of terms of this sequence (A005097) is the set of integers greater than 1, i.e.: 1+1=2, 1+2=3, ..., 5+5=10, ... (This is equivalent to Goldbach's conjecture: every even integer greater than or equal to 6 can be expressed as the sum of two odd primes.) - Lear Young, May 20 2014 See conjecture and comments from Richard R. Forberg, in Links section below, on the relationship of this sequence to rules on values of c that allow both p^q+c and p^q-c to be prime, for an infinite number of primes p. - Richard R. Forberg, Jul 13 2016 The sequence represents the minimum number Ng of gears which are needed to draw a complete graph of order p using a Spirograph®, where p is an odd prime. The resulting graph consists of Ng hypotrochoids whose respective nodes coincide. If the teethed ring has a circumference p then Ng = (p-1)/2. Examples: A complete graph of order three can be drawn with a Spirograph® using a ring with 3n teeth and one gear with n teeth. n is an arbitrary number, only related to the geometry of the gears. A complete graph of order 5 can be drawn using a ring with diameter 5 and 2 gears with diameters 1 and 2 respectively. A complete graph of order 7 can be drawn using a ring with diameter 7 and 3 gears with diameters 1, 2 and 3 respectively. - Bob Andriesse, Mar 31 2017 LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 Richard R. Forberg, Comments on A005097 Peter Horak, Bader F. AlBdaiwi, Diameter Perfect Lee Codes, arXiv:1109.3475 [cs.IT], 2011-2012. Daniel Kohen and Ivan Sadofschi, A New Approach on the Seating Couples Problem, arXiv:1006.2571 [math.CO], 2010. Dhananjay P. Mehendale, On Hamilton Decompositions, arXiv:0806.0251 [math.GM], 2008. Eric Weisstein's World of Mathematics, Legendre Symbol FORMULA a(n) = A006093(n)/2 = A000010(A000040(n+1))/2. a(n) = (prime(n+1)^2-1)/(2*sigma(prime(n+1))) = (A000040(n+1)^2-1)/(2*A000203(A000040(n+1))). - Gary Detlefs, May 02 2012 a(n) = (A065091(n) - 1) / 2. - Reinhard Zumkeller, Jan 02 2013 a(n) = floor(prime(n+1)/2) from Waldemar Puszkarz (see Mathematica field). - Bruno Berselli, Apr 20 2015 a(n) ~ n*log(n)/2. - Ilya Gutkovskiy, Jul 11 2016 a(n) = A294507(n) (mod prime(n+1)). - Jonathan Sondow, Nov 04 2017 MAPLE with(numtheory): p:=n-> ithprime(n):seq((p(n+1)^2-1)/(2*sigma(p(n+1))), n= 1..64) # Gary Detlefs, May 02 2012 MATHEMATICA Table[p=Prime[n]; (p-1)/2, {n, 2, 22}] (* Vladimir Joseph Stephan Orlovsky, Apr 29 2008 *) Table[Floor[Prime[1+n]/Prime], {n, 500}] (* Waldemar Puszkarz, Feb 09 2015 *) PROG (PARI) forprime(p=3, 1e4, print1(p>>1", ")) \\ Charles R Greathouse IV, Jun 16 2011 (Haskell) a005097 = (`div` 2) . a065091 -- Reinhard Zumkeller, Jan 02 2013 (MAGMA) [n: n in [1..160] |IsPrime(2*n+1)]; // Vincenzo Librandi, Feb 16 2015 CROSSREFS Complement of A047845. Cf. A000040, A006005, A006093. A130290 is an essentially identical sequence. Cf. A005384 (subsequence of primes), A266400 (their indices in this sequence). Numbers n such that 2n+k is prime: this seq(k=1), A067076 (k=3), A089038 (k=5), A105760 (k=7), A155722 (k=9), A101448 (k=11), A153081 (k=13), A089559 (k=15), A173059 (k=17), A153143 (k=19). Numbers n such that 2n-k is prime: A006254 (k=1), A098090 (k=3), A089253 (k=5), A089192 (k=7), A097069 (k=9), A097338 (k=11), A097363 (k=13), A097480 (k=15), A098605 (k=17), A097932 (k=19). Cf. also A266409, A294507. Sequence in context: A082583 A274332 A130290 * A102781 A139791 A027563 Adjacent sequences:  A005094 A005095 A005096 * A005098 A005099 A005100 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified October 21 22:47 EDT 2019. Contains 328315 sequences. (Running on oeis4.)