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A002859
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a(1)=1, a(2)=3; for n >= 3, a(n) is smallest number which is uniquely of the form a(j)+a(k) with 1<=j<k<n.
(Formerly M2303 N0909)
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10
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1, 3, 4, 5, 6, 8, 10, 12, 17, 21, 23, 28, 32, 34, 39, 43, 48, 52, 54, 59, 63, 68, 72, 74, 79, 83, 98, 99, 101, 110, 114, 121, 125, 132, 136, 139, 143, 145, 152, 161, 165, 172, 176, 187, 192, 196, 201, 205, 212, 216, 223, 227, 232, 234, 236, 243, 247, 252, 256, 258
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OFFSET
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1,2
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COMMENTS
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An Ulam-type sequence - see A002858 for many further references, comments, etc.
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REFERENCES
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S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 145-151.
R. K. Guy, Unsolved Problems in Number Theory, Section C4.
R. K. Guy, ``s-Additive sequences,'' preprint, 1994.
C. Pickover, Mazes for the Mind, St. Martin's Press, NY, 1992, p. 358.
R. Queneau, Sur les suites s-additives, J. Combin. Theory, A12 (1972), 31-71.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
S. M. Ulam, Problems in Modern Mathematics, Wiley, NY, 1960, p. ix.
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..10000
S. R. Finch, Ulam s-Additive Sequences
Eric Weisstein's World of Mathematics, Ulam Sequence
Wikipedia, Ulam number
Index entries for Ulam numbers
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EXAMPLE
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7 is missing since 7=1+6=3+4; but 8 is present since 8=3+5 has a unique representation.
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MATHEMATICA
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s = {1, 3}; Do[ AppendTo[s, n = Last[s]; While[n++; Length[ DeleteCases[ Intersection[s, n-s], n/2, 1, 1]] != 2]; n], {60}]; s (* From Jean-François Alcover, Oct 20 2011 *)
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PROG
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(Haskell)
a002859 n = a002859_list !! (n-1)
a002859_list = 1 : 3 : ulam 2 3 a002859_list
-- Function ulam as defined in A002858.
-- Reinhard Zumkeller, Nov 03 2011
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CROSSREFS
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Cf. A199118, A199119.
Sequence in context: A051916 A130216 A120162 * A180646 A062514 A065875
Adjacent sequences: A002856 A002857 A002858 * A002860 A002861 A002862
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KEYWORD
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nonn,nice
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AUTHOR
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N. J. A. Sloane, Mira Bernstein
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STATUS
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approved
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