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 A117640 #23 Apr 19 2023 17:47:25
%S A117640 1,12,123,12310,1231011,123101112,12310111213,1231011121320,
%T A117640 123101112132021,12310111213202122,1231011121320212223,
%U A117640 123101112132021222330,12310111213202122233031
%N A117640 Concatenation of first n numbers in base 4.
%C A117640 Concatenation of the first n terms of A007090.
%C A117640 Base-4 analog of A058935.
%H A117640 N. J. A. Sloane, <a href="/A117640/b117640.txt">Table of n, a(n) for n = 1..35</a>
%t A117640 Table[FromDigits[Flatten[Table[IntegerDigits[n,4],{n,k}]]],{k,15}] (* _Harvey P. Dale_, Jan 18 2023 *)
%o A117640 (Python)
%o A117640 from gmpy2 import digits
%o A117640 def A117640(n): return int(''.join(digits(n,4) for n in range(1,n+1))) # _Chai Wah Wu_, Apr 19 2023
%Y A117640 Cf. A030373, A048436.
%Y A117640 Other bases: A058935 (2), A360502 (3), A007908 (10).
%K A117640 base,easy,nonn
%O A117640 1,2
%A A117640 _Jonathan Vos Post_, Apr 27 2006
%E A117640 Edited by _Jason Kimberley_, Nov 27 2012