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 A171828 #24 Oct 12 2024 01:53:50
%S A171828 0,1,2,3,4,5,6,7,8,9,10,11,12,14,15,16,19,20,21,23,24,25,26,27,29,30,
%T A171828 31,35,40,45,49,50,51,55,60,70,75,80,95,100,105,115,120,124,125,126,
%U A171828 130,135,145,150,155,175,200,225,245,250,255,275,300,375,500,600,620,630,650,750,1250,3125
%N A171828 Nonnegative integers that can be made by using five fives (5 5's) and the four basic operators {+, -, *, /}.
%C A171828 Every integer can be made if other operators are allowed (i.e., 13 = (5!+5+5)/(5+5)). The sequence is finite: a(68) = 5*5*5*5*5 = 3125 is the last term.
%H A171828 Wikipedia, <a href="http://en.wikipedia.org/wiki/Four_fours">Four Fours</a>
%e A171828 Example: a(14) = 14 = (5+5+5)-(5/5).
%o A171828 (PARI) A171828(n=5, S=Vec([[n]], n))={for(n=2, n, S[n]=Set(concat(vector(n\2, k, concat([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])} \\ A171828() yields this sequence, use optional arg to compute variants. - _M. F. Hasler_, Nov 24 2018
%Y A171828 Cf. A171826, A171827, A171829, A258068, A258069, A258070, A258071.
%Y A171828 Cf. A182002, A258097.
%K A171828 nonn,fini,full
%O A171828 1,3
%A A171828 _Sergio Pimentel_, Dec 19 2009