cp's OEIS Frontend

A338744 When a(n) is even, a(n) is the number of odd digits present so far in the sequence, a(n) included.

%I A338744 #9 Nov 15 2020 12:55:42
%S A338744 0,1,3,2,5,7,4,9,11,13,10,15,17,19,21,18,23,25,20,27,29,22,31,24,33,
%T A338744 26,35,28,37,39,41,34,43,36,45,38,47,49,40,51,42,53,44,55,46,57,48,59,
%U A338744 61,52,63,54,65,56,67,58,69,71,73,75,77,79,70,81,72,83,74,85,76,87,78,89,91,93,95,97,99,90,101
%N A338744 When a(n) is even, a(n) is the number of odd digits present so far in the sequence, a(n) included.
%C A338744 The odd nonnegative integers are present in their natural order. Some even natural integers will never appear (6 for instance).
%e A338744 The first even term is a(1) = 0 and there is indeed 0 odd digit so far in the sequence;
%e A338744 The next even term is a(4) = 2 and there are now 2 odd digits so far (1 and 3);
%e A338744 The next even term is a(7) = 4 and there are now 4 odd digits so far (1, 3, 5 and 7);
%e A338744 ...
%e A338744 The even term a(11) = 10 and there are indeed 10 odd digits in the sequence so far (1, 3, 5, 7, 9, 1, 1, 1, 3 and 1); etc.
%t A338744 Block[{a = {0}, c = 0}, Do[Block[{k = 1, s}, While[If[EvenQ[k], Nand[FreeQ[a, k], k == c + Set[s, Total@ DigitCount[k, 10, {1, 3, 5, 7, 9}]]], ! FreeQ[a, k]], k++]; If[EvenQ[k], c += s, c += Total@ DigitCount[k, 10, {1, 3, 5, 7, 9}]]; AppendTo[a, k]], {i, 78}]; a] (* _Michael De Vlieger_, Nov 06 2020 *)
%Y A338744 Cf. A338741, A338742, A338743, A338745, A338746 (variants on the same idea), A196564.
%K A338744 base,nonn
%O A338744 1,3
%A A338744 _Eric Angelini_ and _Carole Dubois_, Nov 06 2020