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 A123509 Rohrbach's problem: a(n) is the largest integer such that there exists a set of n integers that is a basis of order 2 for (0, 1, ..., a(n)-1). 5

%I

%S 1,3,5,9,13,17,21,27,33,41,47,55

%N Rohrbach's problem: a(n) is the largest integer such that there exists a set of n integers that is a basis of order 2 for (0, 1, ..., a(n)-1).

%C Notation: N[q] = the set of q+1 elements inside {0,1,...,N-1}

%C Length of the longest sequence of consecutive integers that can be obtained from a set of n distinct integers by summing any two integers in the set or doubling any one. - _Jon E. Schoenfield_, Jul 16 2017

%C According to Zhining Yang, Jul 08 2017, a(13) to a(20) are 65, 70, 79, 90, 101, 112, 123, 134, but there is some doubt about these terms, and they should be confirmed before they are accepted. They do not agree with the conjecture, so perhaps the VBA program is not correct.

%C The definition of Rohrbach's Problem in the paper of S. Gunturk and M. B. Nathanson in the links is different from the one here. In the paper, the set should contain n nonnegative integers instead of integers. The result should be equal to A001212(n-1)+1 according to the definition in the paper since adding one 0 before any set for A001212(n-1) provides a set of the problem. The data provided by Zhining Yang is obviously wrong since a(n) >= A001212(n-1)+1. And A302648 provides another lower bound of this array since a(n) >= 2*A302648(n)+1. - _Zhao Hui Du_, Apr 13 2018

%H S. Gunturk and M. B. Nathanson, <a href="http://dx.doi.org/10.4064/aa124-3-3">A new upper bound for finite additive bases</a>, Acta Arithmetica, Vol. 124, No. 3 (2006) 235-255.

%H H. Rohrbach, <a href="http://www.digizeitschriften.de/dms/resolveppn/?PID=GDZPPN002377063">Ein Beitrag zur additive Zahlentheorie</a>, Math. Z. 42 (1937) 1-30.

%H Kagawa, <a href="http://club.excelhome.net/forum.php?mod=redirect&amp;goto=findpost&amp;ptid=1355454&amp;pid=9161208">VBA program</a>

%F a(n) = A001212(n-1)+1 (conjecture). - _R. J. Mathar_, Oct 08 2006. Comment from _Martin Fuller_, Mar 18 2009: I agree with this conjecture.

%F lim inf a(n) / n^2 > 0.2857 lim sup a(n) / n^2 < 0.4789 - _Charles R Greathouse IV_, Aug 11 2007

%e Example: 8: 0,1,3,4 means {0,1,2,...,8} is covered thus: 0=0+0, 1=0+1, 2=1+1, 3=0+3, 4=0+4=1+3, 5=1+4, 6=3+3, 7=3+4, 8=4+4.

%e N[q]: set

%e ------------------------------

%e 3: 0,1,

%e 4: 0,1,2,

%e 5: 0,1,2,

%e 6: 0,2,3,

%e 7: 0,1,2,3,

%e 8: 0,1,3,4,

%e 9: 0,1,3,4,

%e 10: 0,1,2,4,5,

%e 11: 0,1,2,4,5,

%e 12: 0,1,3,5,6,

%e 13: 0,1,3,5,6,

%e 14: 0,1,2,4,6,7,

%e 15: 0,1,2,4,6,7,

%e 16: 0,1,3,5,7,8,

%e 17: 0,1,3,5,7,8,

%e 18: 0,2,3,7,8,10,

%e 19: 0,1,2,4,6,8,9,

%e 20: 0,1,3,5,7,9,10,

%e 21: 0,1,3,5,7,9,10,

%e 22: 0,2,3,7,8,10,11,

%e 23: 0,1,2,4,6,8,10,11,

%e 24: 0,1,3,5,7,9,11,12,

%e 25: 0,1,3,5,7,9,11,12,

%e 26: 0,2,3,7,8,10,12,13,

%e 27: 0,1,3,4,9,10,12,13,

%e 28: 0,2,3,7,8,12,13,15,

%e 29: 0,1,3,5,7,9,11,13,14,

%e 30: 0,2,3,7,8,10,12,14,15,

%e 31: 0,1,3,4,9,10,12,14,15,

%e 32: 0,2,3,7,8,12,13,15,16,

%e a(5)=13 because we can obtain at most a total of 13 consecutive integers from a set of 5 integers by summing any two integers in the set or doubling any one; from the 5-integer set {1,2,4,6,7}, we can obtain all 13 integers in the interval [2..14] as follows: 2=1+1, 3=1+2, 4=2+2, 5=1+4, 6=2+4, 7=1+6, 8=2+6, 9=2+7, 10=4+6, 11=4+7, 12=6+6, 13=6+7, 14=7+7.

%e a(16)=90 because we can obtain at most a total of 90 consecutive integers from a set of 16 integers by summing any two integers in the set or doubling any one: from the 16-integer set {1,2,4,5,8,9,10,17,18,22,25,36,47,58,69,80}, we can obtain all 90 integers in the interval [2..91]. - _Jon E. Schoenfield_, Jul 16 2017

%Y Cf. A001212, A008932.

%K hard,more,nonn

%O 1,2

%A _Warren D. Smith_, Oct 02 2006

%E More terms (from Smith's web site) from _R. J. Mathar_, Oct 08 2006

%E Entry revised by _N. J. A. Sloane_, Aug 06 2017

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