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 A258070 #13 Nov 26 2018 14:43:51
%S A258070 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,
%T A258070 26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,
%U A258070 49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67
%N A258070 Nonnegative integers that can be computed using exactly nine 9's and the four basic arithmetic operations {+, -, *, /}.
%C A258070 The smallest non-computable number here is 195. The largest computable number here is 9^9 = 387420489.
%H A258070 Alois P. Heinz, <a href="/A258070/b258070.txt">Table of n, a(n) for n = 1..4769</a>
%p A258070 f:= proc(n) f(n):= `if`(n=1, {9}, {seq(seq(seq([x+y, x-y, x*y,
%p A258070 `if`(y=0, [][], x/y)][], y=f(n-j)), x=f(j)), j=1..n-1)})
%p A258070 end:
%p A258070 sort([select(z->z>=0 and is(z, integer), f(9))[]])[];
%o A258070 (PARI) A258070(n=9, S=Vec([[n]], n))={for(n=2, n, S[n]=Set(concat(vector(n\2, k, Set(concat([Set(concat([[T+U, T-U, U-T, if(U, T/U), if(T, U/T), T*U] | T <- S[n-k]])) | U <- S[k]])))))); select(t->t>=0 && type(t)=="t_INT", S[n])} \\ Requires at least 30 MB stack. (Use allocatemem()). A258070() yields this sequence, use optional arg to compute variants. - _M. F. Hasler_, Nov 26 2018
%Y A258070 Cf. A171826, A171827, A171828, A171829, A258068, A258069, A258071.
%Y A258070 Cf. A182002, A258097.
%K A258070 nonn,fini,full
%O A258070 1,3
%A A258070 _Alois P. Heinz_, May 18 2015