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 A174911 Sequence by greedy construction satisfying Lucier-Sárközy difference set condition. 2
 1, 4, 9, 12, 33, 36, 57, 60, 65, 68, 119, 122, 209, 212, 217, 220, 623, 626, 713, 716, 721, 724, 745, 748, 897, 900, 987, 990, 2561, 2564, 2779, 2782, 3807, 3810, 3891, 3894, 4199, 4202, 4585, 4588, 5339, 5342, 5459, 5462, 5963, 5966, 8643, 8646, 12085, 12088 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Ruzsa proves that if N is an integer and A contained in {1, . . . ,N} is such that the difference between any two elements of A is never one less than a prime, then |A| = O(N exp(-c*(log N)^(1/4))) for some absolute c > 0. REFERENCES J. Lucier. Difference sets and shifted primes. Acta Math. Hungar., 120(1-2):79-102, 2008. I. Z. Ruzsa. On measures on intersectivity. Acta Math. Hungar., 43(3-4):335-340, 1984. A. Sárközy. On difference sets of sequences of integers. III. Acta Math. Acad. Sci. Hungar., 31(3-4):355-386, 1978. LINKS Alois P. Heinz, Table of n, a(n) for n = 1..274 Imre Z. Ruzsa, Tom Sanders, Difference sets and the primes, April 1, 2010. FORMULA a(1) = 1. a(n) = least positive integer k such that the difference between any two elements of [a(1), ..., a(n-1)} is never one less than a prime. EXAMPLE a(1) = 1 by definition. a(2) cannot be 2 because 2-a(1)=2-1=1 which is 1 less than 2=prime(1). a(2) cannot be 3 because 3-a(1)=3-1=2 which is 1 less than 3=prime(2). a(2) = 4 because the next smallest integer 4 is such that 4-1=3 and 3+1 is not prime. Next, a(3) cannot be 5 or 6 because as above, an increment of 1 or 2 above the previous value does not work. a(3) cannot be 8 because 8-4=4 and 4+1 is 5 = prime(3). However, a(3)=9 because 9-1=8 (not 1 less than a prime) and 9-4=5 (not 1 less than a prime). MAPLE A174911 := proc(n) option remember ; local wrks, a, i; if n = 1 then 1; elif n = 2 then 4; else for a from procname(n-1)+1 do wrks := true; for i from 1 to n-1 do if isprime(abs(a-procname(i))+1) then wrks := false; break; end if; end do; if wrks then return a; end if; end do: end if: end proc: seq(A174911(n), n=1..80) ; # R. J. Mathar, Apr 15 2010 MATHEMATICA a[1] = 1; a[n_] := a[n] = For[k = 2, True, k++, If[FreeQ[aa = Array[a, n-1], k] && AllTrue[Abs[k-aa], !PrimeQ[#+1]&], Return[k]]]; Array[a, 50] (* Jean-François Alcover, Nov 07 2017 *) CROSSREFS Cf. A000040, A131849. Sequence in context: A083351 A055381 A287498 * A032820 A197615 A125848 Adjacent sequences:  A174908 A174909 A174910 * A174912 A174913 A174914 KEYWORD nonn AUTHOR Jonathan Vos Post, Apr 01 2010 EXTENSIONS More terms from R. J. Mathar, Apr 15 2010 STATUS approved

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Last modified December 2 23:30 EST 2021. Contains 349445 sequences. (Running on oeis4.)