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 A171826 #20 Oct 12 2024 01:53:53
%S A171826 0,2,3,4,6,9,12,18,27
%N A171826 Nonnegative integers that can be made using exactly three threes (3 3's) and the four basic operators {+, -, *, /}.
%C A171826 More integers can be made using 3 3's if more operators are allowed (i.e., 1 = 3^(3-3) or 5 = 3! - 3/3). Note that 1 = 3/3 but the sequence requires to use exactly three threes and it's not possible to "get rid" of the third three.
%H A171826 Wikipedia, <a href="http://en.wikipedia.org/wiki/Four_fours">Four Fours</a>
%e A171826 a(1) = 0 = (3-3)*3, a(2) = 2 = 3 - 3/3, a(3) = 3 = 3*3/3,
%e A171826 a(4) = 4 = 3 + 3/3, a(5) = 6 = 3*3 - 3, a(6) = 9 = 3 + 3 + 3,
%e A171826 a(7) = 12 = 3*3 + 3, a(8) = 18 = (3+3)*3, a(9) = 27 = 3*3*3.
%o A171826 (PARI) vecextract([0..27],134484573) \\ Better programs to compute this are available in the related sequences, but they are much longer than this sequence itself. - _M. F. Hasler_, Nov 24 2018
%Y A171826 Cf. A171827, A171828, A171829, A258068, A258069, A258070, A258071 (analog for four 4's, ..., ten 10's).
%Y A171826 Cf. A182002, A258097.
%K A171826 nonn,fini,full
%O A171826 1,2
%A A171826 _Sergio Pimentel_, Dec 19 2009