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 A024639 #34 Jul 31 2025 03:51:21
%S A024639 0,1,2,3,4,5,6,20,21,22,23,24,25,26,40,41,42,43,44,45,46,60,61,62,63,
%T A024639 64,65,66,210,211,212,213,214,215,216,230,231,232,233,234,235,236,250,
%U A024639 251,252,253,254,255,256,400,401,402,403,404,405,406,420,421,422,423,424,425
%N A024639 n written in fractional base 7/2.
%C A024639 To represent a number in base 7, if a digit exceeds 6, subtract 7 and carry 1. In the fractional base 7/2, subtract 7 and carry 2.
%H A024639 Amiram Eldar, <a href="/A024639/b024639.txt">Table of n, a(n) for n = 0..10000</a>
%H A024639 <a href="/index/Ba#base_fractional">Index entries for sequences related to fractional bases</a>.
%e A024639 From _Alonso del Arte_, Apr 21 2019: (Start)
%e A024639 The integers 0 through 6 are written with the digits 0 through 6.
%e A024639 Then, since b = 7/2 is written as 10, and 7 is twice 7/2, 7 is 20 in base 7/2, and therefore a(7) = 20.
%e A024639 a(28) = 210 since 2 * (7/2)^2 + 1 * (7/2) = 2 * 49/4 + 1 * 7/2 = 98/4 + 14/4 = 112/4 = 28. (End)
%t A024639 Select[Table[FromDigits[IntegerDigits[n, 7]], {n, 0, 230}], IntegerQ[FromDigits[IntegerDigits[#], 7/2]] &] (* _Alonso del Arte_, Apr 21 2019 *)
%t A024639 a[n_] := a[n] = If[n == 0, 0, 10 * a[2 * Floor[n/7]] + Mod[n, 7]]; Array[a, 50, 0] (* _Amiram Eldar_, Jul 31 2025 *)
%o A024639 (PARI) a(n) = if(n == 0, 0, 10 * a(n\7 * 2) + n % 7); \\ _Amiram Eldar_, Jul 31 2025
%Y A024639 Cf. A000079, A000420, A007093, A245342.
%K A024639 nonn,base,easy
%O A024639 0,3
%A A024639 _David W. Wilson_