A117140 a(1)=5, a(2)=7; for n >= 3, a(n) is smallest number which is uniquely of the form a(j)+a(k) with 1<=j<k<n.

5, 7, 12, 17, 19, 22, 26, 27, 32, 33, 36, 37, 40, 42, 46, 50, 56, 60, 66, 70, 71, 74, 81, 85, 91, 99, 101, 132, 138, 140, 150, 154, 156, 158, 160, 179, 181, 185, 189, 195, 205, 209, 233, 234, 236, 240, 248, 258, 264, 268, 287, 289, 293, 297, 303, 336, 340, 342, 348

Offset

1,1

Comments

An Ulam-type sequence - see A002858 for many further references, comments, etc.

Does not seem to have any nice periodicity like A007300 does. - Joshua Zucker, May 24 2006

Links

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

A. N. Debono, Ulam Sequences

Eric Weisstein's World of Mathematics, Ulam Sequence

Wikipedia, Ulam number

Index entries for Ulam numbers

Mathematica

max=100; A117140 = {5, 7}; Do[ AppendTo[ A117140, n = Last[A117140]; While[n++; Length[ DeleteCases[ Intersection[A117140, n - A117140], n/2, 1, 1]] != 2]; n], {max}]; A117140 (* Jean-François Alcover, Nov 30 2011 *)

Prog

(Haskell)
a117140 n = a117140_list !! (n-1)
a117140_list = 5 : 7 : ulam 2 7 a117140_list
-- Function ulam as defined in A002858.
-- Reinhard Zumkeller, Nov 03 2011

Crossrefs

Sequence in context: A314308 A314309 A314310 * A263536 A314311 A031144

Adjacent sequences: A117137 A117138 A117139 * A117141 A117142 A117143

Keyword

nonn

Author

N. J. A. Sloane, Apr 29 2006

Extensions

More terms from Joshua Zucker, May 24 2006

Status

approved