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A003666 a(n) is smallest number which is uniquely a(j)+a(k), j<k.
(Formerly M3237)
7
1, 4, 5, 6, 7, 8, 10, 16, 18, 19, 21, 31, 32, 33, 42, 46, 56, 57, 66, 70, 79, 82, 91, 96, 104, 105, 107, 116, 129, 130, 131, 141, 158, 165, 168, 179, 180, 182, 191, 204, 205, 206, 216, 217, 218, 219, 229, 230, 244, 256, 266, 267, 268, 281, 290, 315, 316, 317, 318, 328 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

An Ulam-type sequence - see A002858 for many further references, comments, etc. - T. D. Noe, Jan 21 2008

REFERENCES

R. K. Guy, ``s-Additive sequences,'' preprint, 1994.

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

LINKS

T. D. Noe, Table of n, a(n) for n=1..10000

S. R. Finch, Patterns in 1-additive sequences, Experimental Mathematics 1 (1992), 57-63.

R. K. Guy, s-Additive sequences, Preprint, 1994. (Annotated scanned copy)

Eric Weisstein's World of Mathematics, Ulam Sequence

Wikipedia, Ulam number

Index entries for Ulam numbers

MATHEMATICA

Nest[Append[#, SelectFirst[Union@ Select[Tally@ Map[Total, Select[Permutations[#, {2}], #1 < #2 & @@ # &]], Last@ # == 1 &][[All, 1]], Function[k, FreeQ[#, k]]]] &, {1, 4}, 58] (* Michael De Vlieger, Nov 16 2017 *)

PROG

(Haskell)

a003666 n = a003666_list !! (n-1)

a003666_list = 1 : 4 : ulam 2 4 a003666_list

-- Function ulam as defined in A002858.

-- Reinhard Zumkeller, Nov 03 2011

CROSSREFS

Cf. A003662, A199120, A199121.

Sequence in context: A078778 A037356 A139199 * A295742 A047314 A298670

Adjacent sequences:  A003663 A003664 A003665 * A003667 A003668 A003669

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Mira Bernstein

STATUS

approved

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Last modified August 25 10:20 EDT 2019. Contains 326324 sequences. (Running on oeis4.)