1,3

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 (peter(AT)luschny.de) Feb 28 2005

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

J.-C. Bermond, Graceful graphs, radio antennae and French windmills, pp. 18-37 of R. J. Wilson, editor, Graph Theory and Combinatorics. Pitman, London, 1978.

J. Leech, On the representation of $1,2,\cdots,n$ by differences. J. London Math. Soc. 31 (1956), 160-169.

J. C. P. Miller, Difference bases: Three problems in additive number theory, pp. 299-322 of A. O. L. Atkin and B. J. Birch, editors, Computers in Number Theory. Academic Press, NY, 1971.

B. Wichmann. A note on restricted difference bases. J. London Math.Soc. 38, 1962, 465-466

Klaus Nagel, Evaluation of perfect rulers. C program.

Peter Luschny, Perfect Rulers.

Peter Luschny, Optimal Wichmann Rulers.

Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.

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.

See Klaus Nagel link.

Cf. A103294, A103298, A103299, A103296, A102508.

A080060 is an erroneous version of the sequence, given in Bermond's paper. Cf. A005488.

Sequence in context: A024190 A004116 A004129 this_sequence A080060 A004131 A032782

Adjacent sequences: A004134 A004135 A004136 this_sequence A004138 A004139 A004140

nonn,nice,hard

N. J. A. Sloane (njas(AT)research.att.com), Simon Plouffe (simon.plouffe(AT)gmail.com)

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 (dean.hickerson(AT)yahoo.com), Jan 26 2003

Terms 79,..,123 from Peter Luschny (peter(AT)luschny.de), Feb 28 2005, with verification by an independent program written by Klaus Nagel (nagel.klaus(AT)t-online.de). Using this program Hugo Pfoertner (hugo(AT)pfoertner.org) found the next term 138.

Using this program Hugo Pfoertner (hugo(AT)pfoertner.org) found further evidence for the conjectured term a(21)=153, Feb 23 2005.