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 A258068 #15 Nov 26 2018 14:45:56
%S A258068 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 A258068 26,27,28,29,30,31,32,33,34,35,36,37,39,40,41,42,43,44,45,46,47,48,49,
%U A258068 50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,69,70,71
%N A258068 Nonnegative integers that can be computed using exactly seven 7's and the four basic arithmetic operations {+, -, *, /}.
%C A258068 The smallest non-computable number here is 38. The largest computable number here is 7^7 = 823543.
%H A258068 Alois P. Heinz, <a href="/A258068/b258068.txt">Table of n, a(n) for n = 1..536</a>
%p A258068 f:= proc(n) f(n):= `if`(n=1, {7}, {seq(seq(seq([x+y, x-y, x*y,
%p A258068 `if`(y=0, [][], x/y)][], y=f(n-j)), x=f(j)), j=1..n-1)})
%p A258068 end:
%p A258068 sort([select(z->z>=0 and is(z, integer), f(7))[]])[];
%o A258068 (PARI) A258068(n=7, 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])} \\ A258068() yields this sequence, use optional arg to compute variants. - _M. F. Hasler_, Nov 24 2018
%Y A258068 Cf. A171826, A171827, A171828, A171829, A258069, A258070, A258071.
%Y A258068 Cf. A182002, A258097.
%K A258068 nonn,fini,full
%O A258068 1,3
%A A258068 _Alois P. Heinz_, May 18 2015