cp's OEIS Frontend

A338741 When a(n) is odd, a(n) is the number of odd digits present so far in the sequence, a(n) included.

%I A338741 #10 Nov 15 2020 12:55:08
%S A338741 0,1,2,4,6,8,10,3,12,5,14,7,16,9,11,13,15,17,18,20,22,24,26,28,30,32,
%T A338741 21,34,23,36,25,38,27,40,42,44,46,48,50,29,31,33,35,37,39,52,41,54,43,
%U A338741 56,45,58,47,60,62,64,66,68,70,49,51,53,55,57,59,72,61,74,63,76,65,78,67,80,82,84,86,88,90,69
%N A338741 When a(n) is odd, a(n) is the number of odd digits present so far in the sequence, a(n) included.
%C A338741 The even nonnegative integers are present in their natural order. Some odd natural integers will never appear (19 for instance).
%e A338741 The first odd term is a(2) = 1 and there is indeed 1 odd digit so far in the sequence (1 itself);
%e A338741 The next odd term is a(8) = 3 and there are now 3 odd digits so far (1, 1 and 3);
%e A338741 The next odd term is a(10) = 5 and there are now 5 odd digits so far (1, 1, 3, 1 and 5);
%e A338741 ...
%e A338741 The next odd term is a(18) = 17 and there are indeed 17 odd digits so far in the sequence (1, 1, 3, 1, 5, 1, 7, 1, 9, 1, 1, 1, 3, 1, 5, 1, 7); etc.
%t A338741 Block[{a = {0}, c = 0}, Do[Block[{k = 1, s}, While[If[OddQ[k], Nand[FreeQ[a, k], k == c + Set[s, Total@ DigitCount[k, 10, {1, 3, 5, 7, 9}]]], ! FreeQ[a, k]], k++]; If[OddQ[k], c += s, c += Total@ DigitCount[k, 10, {1, 3, 5, 7, 9}]]; AppendTo[a, k]], {i, 79}]; a] (* _Michael De Vlieger_, Nov 06 2020 *)
%Y A338741 Cf. A338742, A338743, A338744, A338745, A338746 (variants on the same idea), A196564.
%K A338741 base,nonn
%O A338741 1,3
%A A338741 _Eric Angelini_ and _Carole Dubois_, Nov 06 2020