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 A248556 #17 Sep 08 2022 08:46:10
%S A248556 123,456,789,101112,131415,161718,192021,222324,252627,282930,313233,
%T A248556 343536,373839,404142,434445,464748,495051,525354,555657,585960,
%U A248556 616263,646566,676869,707172,737475,767778,798081,828384,858687,888990,919293,949596,979899
%N A248556 Concatenate (3n-2,3n-1,3n).
%p A248556 conc3:= (a,b,c) -> c + 10^(1+ilog10(c))*(b+10^(1+ilog10(b))*a):
%p A248556 seq(conc3(3*k-2,3*k-1,3*k), k=1..100); # _Robert Israel_, Oct 23 2014
%t A248556 FromDigits[Flatten[IntegerDigits/@{#}]]&/@Partition[Range[100], 3] (* _Vincenzo Librandi_, Oct 21 2014 *)
%o A248556 (Magma) [Seqint(Intseq(n+1) cat Intseq(n) cat Intseq(n-1)): n in [2..100 by 3]]; // _Vincenzo Librandi_, Oct 21 2014
%o A248556 (PARI) a(n)=eval(Str(3*n-2,3*n-1,3*n)) \\ _M. F. Hasler_, Oct 22 2014
%Y A248556 Cf. A030655, A091331.
%K A248556 nonn,base,easy
%O A248556 1,1
%A A248556 _Charles Duvall_, Oct 08 2014
%E A248556 More terms from _Vincenzo Librandi_, Oct 21 2014