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A065825 Smallest k such that n numbers may be picked in {1,...,k} with no three terms in arithmetic progression. 15
1, 2, 4, 5, 9, 11, 13, 14, 20, 24, 26, 30, 32, 36, 40, 41, 51, 54, 58, 63, 71, 74, 82, 84, 92, 95, 100, 104, 111, 114, 121, 122, 137, 145, 150, 157, 163, 165, 169, 174, 194 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

"Sequences containing no 3-term arithmetic progressions" is another phrase people may be searching for. See also A003002.

Don Reble notes large gaps between a(4k) and a(4k+1).

Ed Pegg Jr conjectures the 2^k term always equals (3^k+1)/2 and calls these "unprogressive" sets. Jaroslaw Wroblewski (jwr(AT)math.uni.wroc.pl), Nov 04 2003, remarks that this conjecture is known to be false.

Further comments from Jaroslaw Wroblewski (jwr(AT)math.uni.wroc.pl), Nov 05 2003: log a(n) / log n tends to 1 was established in 1946 by Behrend. This was extended by me in the Math. Comp. paper. Using appropriately chosen intervals from B(4,9,4) and B(6,9,11) I have determined yesterday that log (2a(n)-1) / log n < log 3 / log 2 holds for n=60974 and for n=2^19 since a(60974) <= 19197041, a(524288) <= 515749566. See my web page for further bounds.

REFERENCES

Tanbir Ahmed, Janusz Dybizbanski and Hunter Snevily, Unique Sequences Containing No k-Term Arithmetic Progressions, Electronic Journal of Combinatorics, 20(4), 2013, #P29.

F. Behrend, On sets of integers which contain no three terms in an arithmetic progression, Proc. Nat. Acad. Sci. USA, v. 32, 1946, pp. 331-332.

Janusz Dybizbanski, Sequences containing no 3-term arithmetic progressions, Electronic Journal of Combinatorics, 19(2), 2012, #P15.

C. R. J. Singleton, "No Progress": Solution to Problem 2472, Journal of Recreational Mathematics, 30(4) 305 1999-2000.

S. S. Wagstaff, Jr., On k-free sequences of integers, Math. Comp., 26 (1972), 767-771.

J. Wroblewski, A Nonaveraging Set of Integers With a Large Sum of Reciprocals, Mathematics of Computation, Volume 43, Number 167, 1984, pp. 261-262.

LINKS

Table of n, a(n) for n=1..41.

J. Wroblewski, Nonaveraging Sets

Tom Sanders, On Roth’s theorem on progressions, Annals of Mathematics 174:1 (2011), pp. 619-636. arXiv:1011.0104

Index entries related to non-averaging sequences

EXAMPLE

a(9) = 20 = 1 2 6 7 9 14 15 18 20

a(10) = 24 = 1 2 5 7 11 16 18 19 23 24

a(11) = 26 = 1 2 5 7 11 16 18 19 23 24 26

a(12) = 30 = 1 3 4 8 9 11 20 22 23 27 28 30 (unique)

a(13) = 32 = 1 2 4 8 9 11 19 22 23 26 28 31 32

a(14) = 36 = 1 2 4 8 9 13 21 23 26 27 30 32 35 36

a(15) = 40 = 1 2 4 5 10 11 13 14 28 29 31 32 37 38 40

a(16) = 41 = 1 2 4 5 10 11 13 14 28 29 31 32 37 38 40 41

a(17) = 51 = 1 2 4 5 10 13 14 17 31 35 37 38 40 46 47 50 51

a(18) = 54 = 1 2 5 6 12 14 15 17 21 31 38 39 42 43 49 51 52 54

a(19) = 58 = 1 2 5 6 12 14 15 17 21 31 38 39 42 43 49 51 52 54 58

a(20) = 63 = 1 2 5 7 11 16 18 19 24 26 38 39 42 44 48 53 55 56 61 63

a(21) = 71 = 1 2 5 7 10 17 20 22 26 31 41 46 48 49 53 54 63 64 68 69 71

a(22) = 74 = 1 2 7 9 10 14 20 22 23 25 29 46 50 52 53 55 61 65 66 68 73 74

a(23) = 82 = 1 2 4 8 9 11 19 22 23 26 28 31 49 57 59 62 63 66 68 71 78 81 82

a(24) = 84 = 1 3 4 8 9 16 18 21 22 25 30 37 48 55 60 63 64 67 69 76 77 81 82 84

a(25) = 92 = 1 2 6 8 9 13 19 21 22 27 28 39 58 62 64 67 68 71 73 81 83 86 87 90 92

a(26) = 95 = 1 2 4 5 10 11 22 23 25 26 31 32 55 56 64 65 67 68 76 77 82 83 91 92 94 95

a(27) = 100 = 1 3 6 7 10 12 20 22 25 26 29 31 35 62 66 68 71 72 75 77 85 87 90 91 94 96 100

CROSSREFS

Cf. A003002 (three-free sequences), A003278, A003003, A003004, A003005.

A005047(n) + 1. - Rob Pratt, Jul 09 2015

Sequence in context: A287181 A047378 A101155 * A113755 A124254 A192615

Adjacent sequences:  A065822 A065823 A065824 * A065826 A065827 A065828

KEYWORD

hard,nice,more,nonn

AUTHOR

Ed Pegg Jr, Nov 23 2001

EXTENSIONS

a(19) found by Guenter Stertenbrink in response to an A003278-based puzzle on www.mathpuzzle.com

More terms from Don Reble (djr(AT)nk.ca), Nov 25 2001

Five more terms from William Rex Marshall, Mar 24 2002

137 from William Rex Marshall, Nov 15 2003

One more term from William Rex Marshall, Jan 24 2004

a(35)-a(36) (found by Gavin Theobold in 2004) communicated by William Rex Marshall, Mar 10 2007.

Added a(37)-a(41) (from Wroblewski's web page), Joerg Arndt, Apr 25 2012.

STATUS

approved

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Last modified September 26 15:27 EDT 2017. Contains 292531 sequences.