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 A239308 #13 Mar 18 2014 17:44:06
%S A239308 1,2,3,3,4,4,4,5,5,5,6,6,6,6,7,7,7,7,7,8,8,8,8,8,8,9,9,9,9,9,10,10,10,
%T A239308 10,10,10,10,10
%N A239308 Size of smallest set S of integers such that {0,1,2,...,n} is a subset of S-S, where S-S={abs(i-j) | i,j in S}.
%C A239308 S need not be a subset of {0,1,2,...,n}, unlike the definition in A046693.
%D A239308 J. Leech, On the representation of 1,2,...,n by differences, J. London Math. Soc. 31 (1956) 160-169.
%e A239308 a(18)=7 since all integers in {0,1,2...18} are differences of elements of {0,6,9,10,17,22,24}, but not of any 6-element set.
%e A239308 In other words, {0,6,9,10,17,22,24} is an unrestricted difference basis w.r.t. A005488(7)=18.
%Y A239308 A005488, A046693
%K A239308 nonn
%O A239308 0,2
%A A239308 _Steven Finch_, Mar 18 2014