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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

%I A117140 #17 Feb 16 2025 08:33:00
%S A117140 5,7,12,17,19,22,26,27,32,33,36,37,40,42,46,50,56,60,66,70,71,74,81,
%T A117140 85,91,99,101,132,138,140,150,154,156,158,160,179,181,185,189,195,205,
%U A117140 209,233,234,236,240,248,258,264,268,287,289,293,297,303,336,340,342,348
%N 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.
%C A117140 An Ulam-type sequence - see A002858 for many further references, comments, etc.
%C A117140 Does not seem to have any nice periodicity like A007300 does. - _Joshua Zucker_, May 24 2006
%H A117140 T. D. Noe, <a href="/A117140/b117140.txt">Table of n, a(n) for n=1..10000</a>
%H A117140 A. N. Debono, <a href="http://www.eng.um.edu.mt/~andebo/numbers/numcom27.htm">Ulam Sequences</a>
%H A117140 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/UlamSequence.html">Ulam Sequence</a>
%H A117140 Wikipedia, <a href="http://en.wikipedia.org/wiki/Ulam_number">Ulam number</a>
%H A117140 <a href="/index/U#Ulam_num">Index entries for Ulam numbers</a>
%t A117140 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 *)
%o A117140 (Haskell)
%o A117140 a117140 n = a117140_list !! (n-1)
%o A117140 a117140_list = 5 : 7 : ulam 2 7 a117140_list
%o A117140 -- Function ulam as defined in A002858.
%o A117140 -- _Reinhard Zumkeller_, Nov 03 2011
%K A117140 nonn
%O A117140 1,1
%A A117140 _N. J. A. Sloane_, Apr 29 2006
%E A117140 More terms from _Joshua Zucker_, May 24 2006