A344542 Lexicographically earliest sequence S of distinct positive terms such that the product of the last k digits of S is odd, k being the rightmost digit of a(n).

1, 11, 3, 5, 13, 7, 9, 15, 17, 19, 21, 31, 33, 35, 37, 39, 41, 51, 53, 55, 57, 59, 61, 71, 73, 75, 77, 79, 81, 91, 93, 95, 97, 99, 101, 111, 113, 115, 117, 119, 121, 131, 133, 135, 137, 139, 141, 151, 153, 155, 157, 159, 161, 171, 173, 175, 177, 179, 181, 191, 193, 195, 197, 199, 201, 211, 221, 231, 241, 251

Offset

1,2

Comments

No term ends with zero.

Links

Table of n, a(n) for n=1..70.

Example

a(1) = 1, k = 1, and the product 1 is odd;
a(2) = 11, k = 1, and the product 1 is odd;
a(3) = 3, k = 3, and the product 1*1*3 (= 3) is odd;
a(4) = 5, k = 5, and the product 1*1*1*3*5 (= 15) is odd;
a(5) = 13, k = 3, and the product 5*1*3 (= 15) is odd;
a(6) = 7, k = 7, and the product 1*1*3*5*1*3*7 (= 315) is odd; etc.

Mathematica

a[1]=1; a[n_]:=a[n]=Block[{t=1}, While[(k=t~Mod~10; k>Length[s=Flatten[IntegerDigits/@Join[Array[a, n-1], {t}]]])||k<1||MemberQ[Array[a, n-1], t]||EvenQ[Times@@(s[[-k;; ]])], t++]; t]; Array[a, 70] (* Giorgos Kalogeropoulos, May 10 2022 *)

Crossrefs

Cf. A344539.

Sequence in context: A338716 A071234 A082626 * A334132 A245193 A338715

Adjacent sequences: A344539 A344540 A344541 * A344543 A344544 A344545

Keyword

base,nonn

Author

Eric Angelini and Carole Dubois, May 30 2021

Status

approved