This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A139217 #5 Feb 12 2014 18:08:52
%S A139217 1,4,7,13,22,49,97,190,385,769,1534,3073,6145,12286
%N A139217 Smallest positive integer of the form 3k+1 such that all subsets of {a(1),...,a(n)} have a different sum.
%C A139217 (1) It appears that {a(n+1)-2a(n)} is eventually periodic, with values {2,-1,-1,-4,5,-1,-4,5,-1,-4,5,-1,-4,...}.
%C A139217 (2) See A139218 for the corresponding sequence using integers of the form 3k+2.
%C A139217 (3) _M. F. Hasler_, in a SeqFan memo dated Apr 09 2008, notes that the Jacobsthal sequence (A001045) from a(2) on (i.e., 1,3,5,11,21,...) gives the smallest positive odd integer such that all subsets of {a(2),...,a(n)} have a different sum.
%F A139217 It appears that a(n)=a(n-1)+a(n-2)+2a(n-3), for n>4.
%Y A139217 Cf. A001045, A139218.
%K A139217 nonn
%O A139217 1,2
%A A139217 _John W. Layman_, Apr 11 2008