A338743 When a(n) is odd, a(n) is the number of even digits present so far in the sequence, a(n) included.

0, 1, 2, 4, 3, 6, 8, 5, 10, 12, 7, 14, 16, 9, 18, 20, 22, 24, 26, 28, 21, 30, 23, 32, 25, 34, 27, 36, 29, 38, 40, 42, 44, 46, 48, 41, 50, 43, 52, 45, 54, 47, 56, 49, 58, 60, 62, 64, 66, 68, 61, 70, 63, 72, 65, 74, 67, 76, 69, 78, 80, 82, 84, 86, 88, 81, 90, 83, 92, 85, 94, 87, 96, 89, 98, 100, 102, 104

Offset

1,3

Comments

The even nonnegative integers are present in their natural order. Some odd natural integers will never appear (11 for instance).

Links

Table of n, a(n) for n=1..78.

Example

The first odd term is a(2) = 1 and there is indeed 1 even digit so far in the sequence (0);
The next odd term is a(5) = 3 and there are now 3 even digits so far (0, 2 and 4);
The next odd term is a(8) = 5 and there are now 5 even digits so far (0, 2, 4, 6 and 8);
...
The term a(21) = 21 and there are indeed 21 even digits in the sequence so far (0, 2, 4, 6, 8, 0, 2, 4, 6, 8, 2, 0, 2, 2, 2, 4, 2, 6, 2, 8, 2); etc.

Mathematica

Block[{a = {0}, c = 1}, Do[Block[{k = 1, s}, While[If[OddQ[k], Nand[FreeQ[a, k], k == c + Set[s, Total@DigitCount[k, 10, {0, 2, 4, 6, 8}]]], ! FreeQ[a, k]], k++]; If[OddQ[k], c += s, c += Total@ DigitCount[k, 10, {0, 2, 4, 6, 8}]]; AppendTo[a, k]], {i, 77}]; a] (* Michael De Vlieger, Nov 06 2020 *)

Crossrefs

Cf. A338741, A338742, A338744, A338745, A338746 (variants on the same idea), A196563.

Sequence in context: A233342 A120233 A265667 * A358267 A280866 A280864

Adjacent sequences: A338740 A338741 A338742 * A338744 A338745 A338746

Keyword

base,nonn

Author

Eric Angelini and Carole Dubois, Nov 06 2020

Status

approved