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 A288759 #16 Aug 06 2017 13:05:48
%S A288759 1,3,8,38,555,74423,902663448
%N A288759 Number of distinct rational numbers that can be obtained in n steps by applying addition, subtraction, multiplication and division to any two potentially identical numbers from the complete set of numbers created in n-1 steps, starting with the set {1}.
%C A288759 This is different from a straight-line program (SLP), which can only use numbers created in the path to its own result at level n-1. A288849 provides the cardinalities of the sets that can be created by the related SLPs.
%e A288759 The sets of numbers obtainable at the n-th step are:
%e A288759 S(0) = { 1 },
%e A288759 S(1) = { 0, 1, 2 },
%e A288759 S(2) = { -2, -1, 0, 1/2, 1, 2, 3, 4 },
%e A288759 S(3) = { -8, -6, -5, -4, -7/2, -3, -5/2, -2, -3/2, -1, -2/3, -1/2, -1/3, -1/4, 0, 1/8, 1/6, 1/4, 1/3, 1/2, 2/3, 3/4, 1, 4/3, 3/2, 2, 5/2, 3, 7/2, 4, 9/2, 5, 6, 7, 8, 9, 12, 16 }.
%Y A288759 Cf. A214872, A216999, A288760, A288849.
%K A288759 nonn,more,hard
%O A288759 0,2
%A A288759 _Hugo Pfoertner_, Jun 15 2017
%E A288759 a(6) from _Hugo Pfoertner_ and _Markus Sigg_, Aug 06 2017