

A004137


Maximal number of edges in a graceful graph on n nodes.
(Formerly M2526)


14



0, 1, 3, 6, 9, 13, 17, 23, 29, 36, 43, 50, 58, 68, 79, 90, 101, 112, 123, 138, 153, 168, 183, 198, 213
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OFFSET

1,3


COMMENTS

A graph with e edges is 'graceful' if its nodes can be labeled with distinct integers in {0,1,...,e} so that, if each edge is labeled with the absolute difference between the labels of its endpoints, then the e edges have the distinct labels 1, 2, ..., e.
Equivalently, maximum m for which there's a restricted difference basis with respect to m with n elements. A 'difference basis w.r.t. m' is a set of integers such that every integer from 1 to m is a difference between two elements of the set. A 'restricted' difference basis is one in which the smallest element is 0 and the largest is m.
a(n) is also the length of an optimal ruler with n marks. For definitions see A103294. For example a(6)=13 is the length of the optimal rulers with 6 marks, {[0, 1, 6, 9, 11, 13], [0, 2, 4, 7, 12, 13], [0, 1, 4, 5, 11, 13], [0, 2, 8, 9, 12, 13], [0, 1, 2, 6, 10, 13], [0, 3, 7, 11, 12, 13]}. Also n = 1 + A103298(a(n)).  Peter Luschny, Feb 28 2005
If the conjecture is true that an optimal ruler with more than 12 segments is a Wichmann ruler than the sequence continues 232, 251, 270, 289, 308, 327, ...  Peter Luschny, Oct 09 2011 [updated to take the verifications of Robison into account, Oct 01 2015]


REFERENCES

J.C. Bermond, Graceful graphs, radio antennae and French windmills, pp. 1837 of R. J. Wilson, editor, Graph Theory and Combinatorics. Pitman, London, 1978.
R. K. Guy, Modular difference sets and error correcting codes. in: Unsolved Problems in Number Theory, 3rd ed. New York: SpringerVerlag, chapter C10, (2004), 181183.
J. C. P. Miller, Difference bases: Three problems in additive number theory, pp. 299322 of A. O. L. Atkin and B. J. Birch, editors, Computers in Number Theory. Academic Press, NY, 1971.
I. Redéi and A. Rényi, On the representation of integers 1, 2, ..., n by differences, Mat. Sbornik 24 (1949), 385389 (Russian).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

Table of n, a(n) for n=1..25.
L. Egidi and G. Manzini, Spaced seeds design using perfect rulers, Tech. Rep. CS Department Universita del Piemonte Orientale, June 2011.
P. Erdős, A survey of problems in combinatorial number theory, Ann. Discrete Math. 6 (1980), 89115.
P. Erdős and R. Freud, On sums of a Sidonsequence, J. Number Theory 38 (1991), 196205.
P. Erdős and P. Turán, On a problem of Sidon in additive number theory, and on some related problems, J. Lond. Math. Soc. 16 (1941), 212215.
J. Leech, On the representation of 1, 2, ..., n by differences, J. Lond. Math. Soc. 31 (1956), 160169.
S. Lou and Q. Yao, A Chebyshev's type of prime number theorem in a short interval II, HardyRamanujan J. 15 (1992), 133.
Peter Luschny, Perfect Rulers.
Peter Luschny, Wichmann Rulers.
Klaus Nagel, Evaluation of perfect rulers. C program.
O. Pikhurko, Dense edgemagic graphs and thin additive bases, Discrete Math. 306 (2006), 20972107.
O. Pikhurko and T. Schoen, Integer Sets Having the Maximum Number of Distinct Differences, Integers: Electronic journal of combinatorial number theory 7 (2007).
Arch D. Robison, Parallel Computation of Sparse Rulers, Jan 14 2014.
J. Singer, A theorem in finite projective geometry and some applications to number theory, Trans. Amer. Math. Soc. 43 (1938), 37785.
Eric Weisstein's World of Mathematics, Graceful Graph.
B. Wichmann, A note on restricted difference bases, J. Lond. Math. Soc. 38 (1963), 465466.
Al Zimmermann's Programming Contests, Graceful Graphs, September  December 2013.


EXAMPLE

a(7)=17: Label the 7 nodes 0,1,8,11,13,15,17 and include all edges except those from 8 to 15, from 13 to 15, from 13 to 17 and from 15 to 17. {0,1,8,11,13,15,17} is a restricted difference basis w.r.t. 17.
a(21)=153 because there exists a complete ruler (i.e., one that can measure every distance between 1 and 153) with marks [0,1,2,3,7,14,21,28,43,58,73,88,103,118,126,134,142,150,151,152,153] and no complete ruler of greater length with the same number of marks can be found. This ruler is of the type described by B. Wichmann and is conjectured by P. Luschny that it is impossible to beat Wichmann's construction for finding optimal rulers of bigger lengths.


PROG

(C) See Klaus Nagel link.
(Parallel C++) See A. Robison link.


CROSSREFS

Cf. A046693, A103294, A103298, A103299, A103296, A102508.
A080060 is an erroneous version of the sequence, given in Bermond's paper. Cf. A005488.
Sequence in context: A185173 A171662 A235269 * A080060 A004131 A171514
Adjacent sequences: A004134 A004135 A004136 * A004138 A004139 A004140


KEYWORD

nonn,nice,hard,more


AUTHOR

N. J. A. Sloane, Simon Plouffe


EXTENSIONS

Miller's paper gives these lower bounds for the 8 terms from a(15) to a(22): 79, 90, 101, 112, 123, 138, 153, 168.
Edited by Dean Hickerson, Jan 26 2003
Terms 79,..,123 from Peter Luschny, Feb 28 2005, with verification by an independent program written by Klaus Nagel (nagel.klaus(AT)tonline.de). Using this program Hugo Pfoertner found the next term 138.
Using this program Hugo Pfoertner found further evidence for the conjectured term a(21)=153, Feb 23 2005.
Terms a(21) ... a(24) proved by exhaustive search by Arch D. Robison, Hugo Pfoertner, Nov 01 2013
Term a(25) proved by exhaustive search by Arch D. Robison, Peter Luschny, Jan 14 2014


STATUS

approved



