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 A045507 #34 Jul 26 2025 19:44:46
%S A045507 1,12,124,1248,124816,12481632,1248163264,1248163264128,
%T A045507 1248163264128256,1248163264128256512,12481632641282565121024,
%U A045507 124816326412825651210242048,1248163264128256512102420484096,12481632641282565121024204840968192,1248163264128256512102420484096819216384
%N A045507 Concatenate powers of 2.
%D A045507 A. Murthy, Smarandache Notions Journal, Vol. 11 N. 1-2-3 Spring 2000. [_Felice Russo_, Nov 15 1999]
%H A045507 Vincenzo Librandi, <a href="/A045507/b045507.txt">Table of n, a(n) for n = 0..80</a>
%e A045507 a(2) = 124 because it is the concatenation of 2^0, 2^1 and 2^2.
%p A045507 a:= proc(n) option remember;
%p A045507 `if`(n<0, 0, parse(cat(a(n-1), 2^n)))
%p A045507 end:
%p A045507 seq(a(n), n=0..14); # _Alois P. Heinz_, Jul 26 2025
%t A045507 Join[{1}, FromDigits/@Flatten/@IntegerDigits/@Flatten/@Rest[FoldList[List, {1}, 2^Range[25]]]] (* _Vincenzo Librandi_, Aug 10 2017 *)
%o A045507 (Magma) [Seqint(Reverse(&cat[Reverse(Intseq(2^k)): k in [0..n]])): n in [0..17]]; // _Vincenzo Librandi_, Aug 10 2017
%Y A045507 Cf. A000079, A007908.
%K A045507 nonn,base
%O A045507 0,2
%A A045507 _Jeff Burch_