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 A229673 #13 Mar 24 2017 00:47:55 %S A229673 1,3,15,126,1667,31966,828678,27535826,1128945382,55473589278, %T A229673 3193471420236,211517309562652 %N A229673 The number of subsets of nonzero integers of cardinality n, produced as the steps in a computation starting with 1 and using the operations of multiplication, addition, or subtraction. %C A229673 A straight-line program (SLP) is a sequence that starts at 1 and has each entry obtained from two preceding entries by addition, multiplication, or subtraction. The length of the SLP is defined as that of the sequence without the first 1. An SLP is said to be reduced if there is no repetition in the sequence. Two SLPs are considered equivalent if their sequences define the same subset(only difference is sequence order). This sequence gives the number of SLPs with n steps in which 0 does not appear. %C A229673 This sequence can also be thought of as defining the size of the search space that needs to be traversed when trying to compute other SLP related OEIS sequences such as A216999 or A229686. This is because 0 is never needed in the shortest SLP calculation of any other integer. %C A229673 Michael Collier generated independently the values up to 1128945382. %F A229673 a(n) >= 2^(n-1)*(n-1)!. %e A229673 a(1) = 1 and the SLP is (1,2). %e A229673 a(2) = 3 and the SLPs are (1,2,3), (1,2,4), and (1,2,-1). %Y A229673 Cf. A229662, A216999, A173419, A214872, A229686. %K A229673 nonn,hard %O A229673 1,2 %A A229673 _Gil Dogon_, Sep 27 2013