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 A249873 #10 Jan 23 2022 13:53:48 %S A249873 0,1,2,3,4,5,6,7,8,9,20,21,22,23,24,25,26,27,28,29,40,41,42,43,44,45, %T A249873 46,47,48,49,60,61,62,63,64,65,66,67,68,69,80,81,82,83,84,85,86,87,88, %U A249873 89,10,11,12,13,14,15,16,17,18,19,30,31,32,33,34,35,36 %N A249873 In decimal representation: replace each even positioned digit with the digit sum of its double. %H A249873 Reinhard Zumkeller, <a href="/A249873/b249873.txt">Table of n, a(n) for n = 0..10000</a> %H A249873 <a href="/index/De#decimal_expansion">Index entries for sequences related to decimal expansion of n</a> %o A249873 (Haskell) %o A249873 a249873 n = if n == 0 then 0 else 100*a249873 n' + 10*a007953 (2*t) + d %o A249873 where (n', td) = divMod n 100; (t, d) = divMod td 10 %o A249873 (PARI) a(n) = my(d=Vecrev(digits(n))); forstep (i=2, #d, 2, d[i] = sumdigits(2*d[i])); fromdigits(Vecrev(d)); \\ _Michel Marcus_, Jan 23 2022 %Y A249873 Cf. A007953, A093017, A028897. %K A249873 nonn,base %O A249873 0,3 %A A249873 _Reinhard Zumkeller_, Nov 08 2014