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 A181898 #52 Nov 11 2017 16:48:06 %S A181898 10,19,92,417,851,4237,14771,73237,298609 %N A181898 Smallest positive integer which cannot be calculated by an expression containing n binary operators (any of add, subtract, multiply and divide) whose operands are any integer between 1 and 9; parentheses allowed. %H A181898 Derek M. Jones, <a href="/A181898/a181898_1.r.txt">R program for calculating values</a> %H A181898 <a href="/index/Com#complexity">Index to sequences related to the complexity of n</a> %e A181898 a(2)=92 because at least 3 operators are required, e.g., (2*7 + 9)*4. %o A181898 (R) See Jones link. %o A181898 (PARI) first(n)=my(op=[(x,y)->x+y, (x,y)->x-y, (x,y)->y-x, (x,y)->x*y, (x,y)->x/y, (x,y)->y/x], v=vector(n+1), t); v[1]=[1..9]; for(k=2,#v, my(u=[]); for(i=1,(k+1)\2, my(a=v[i],b=v[k-i]); t=Set(concat(apply(f->setbinop(f,a,b), op))); u=concat(u,t)); v[k]=setminus(Set(u), [0])); t=10; for(i=1,#v, while(setsearch(v[i],t), t++); v[i]=t); v \\ _Charles R Greathouse IV_, Jan 09 2017 %Y A181898 Cf. A181957, A181958, A181959, A181960, A005520, A048183 (see text of comment). %K A181898 nonn %O A181898 0,1 %A A181898 _Derek M. Jones_, Apr 03 2012