A338742 When a(n) is even, a(n) is the number of even digits present so far in the sequence, a(n) included.

1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 2, 23, 4, 25, 6, 27, 8, 29, 10, 31, 33, 35, 37, 39, 41, 12, 43, 14, 45, 16, 47, 18, 20, 22, 24, 26, 28, 49, 30, 51, 53, 55, 57, 59, 61, 32, 63, 34, 65, 36, 67, 38, 40, 42, 44, 46, 48, 69, 50, 71, 73, 75, 77, 79, 81, 52, 83, 54, 85, 56, 87, 58, 60, 62, 64, 66, 68, 89

Offset

1,2

Comments

The odd nonnegative integers are present in their natural order. Some even natural integers will never appear.

Links

Table of n, a(n) for n=1..79.

Example

The first even term is a(12) = 2 and there are indeed 2 even digits so far in the sequence (the 2 from 21 and 2 itself);
The next even term is a(14) = 4 and there are now 4 even digits so far (2, 2, 2 and 4);
The next even term is a(16) = 6 and there are now 6 even digits so far (2, 2, 2, 4, 2 and 6); etc.

Mathematica

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, {0, 2, 4, 6, 8}]]], ! FreeQ[a, k]], k++]; If[EvenQ[k], c += s, c += Total@ DigitCount[k, 10, {0, 2, 4, 6, 8}]]; AppendTo[a, k]], {i, 79}]; a] (* Michael De Vlieger, Nov 06 2020 *)

Crossrefs

Cf. A338741, A338743, A338744, A338745, A338746 (variants on the same idea), A196563.

Sequence in context: A356929 A187349 A238704 * A251238 A059547 A064719

Adjacent sequences: A338739 A338740 A338741 * A338743 A338744 A338745

Keyword

base,nonn

Author

Eric Angelini and Carole Dubois, Nov 06 2020

Status

approved