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 A137507 #19 Apr 17 2025 07:03:08
%S A137507 1,3,5,7,9,11,13,15,17,19,300,320,340,360,380,400,420,440,460,480,
%T A137507 14000,14200,14400,14600,14800,15000,15200,15400,15600,15800,1060000,
%U A137507 1062000,1064000,1066000,1068000,1070000,1072000,1074000,1076000,1078000,100800000,100820000
%N A137507 a(n) = 100^[n/10] + 2*n*10^[n/10]: inspired by Engel expansion of Pi.
%H A137507 G. C. Greubel, <a href="/A137507/b137507.txt">Table of n, a(n) for n = 0..1000</a>
%H A137507 <a href="/index/Rec#order_30">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,0,0,0,0,0,0,120,0,0,0,0,0,0,0,0,0,-2100,0,0,0,0,0,0,0,0,0,10000).
%t A137507 Table[100^(Floor[n/10]) + 2*n*10^(Floor[n/10]), {n,0,50}] (* _G. C. Greubel_, Feb 23 2017 *)
%o A137507 (PARI) a(n) = (n + 10^(floor(n/10)))^2 - n^2;
%o A137507 (PARI) vector(30,n, n--; 2*n*10^(n\10) + 100^(n\10)) \\ _M. F. Hasler_, May 02 2008
%o A137507 (Sage) [(n + 10^(floor(n/10)))^2 - n^2 for n in range(0,42)] # _Stefano Spezia_, Apr 16 2025
%Y A137507 Cf. A006784.
%K A137507 nonn,easy
%O A137507 0,2
%A A137507 _Alexander R. Povolotsky_, Apr 23 2008
%E A137507 Edited by _M. F. Hasler_ and _N. J. A. Sloane_, May 02 2008