OFFSET
1,2
COMMENTS
The lexicographically earliest sequence of positive terms such that a(n) is present in 2*a(n+1) is A000351 (the powers of 5).
LINKS
Carole Dubois, Table of n, a(n) for n = 1..270
EXAMPLE
a(1) = 1 is present (as a substring) in 12 [= 3 * a(n+1) = 3 * 4)];
a(2) = 4 is present in 24 (= 3 * 8);
a(3) = 8 is present in 18 (= 3 * 6);
a(4) = 6 is present in 6 (= 3 * 2); etc.
PROG
(Magma) a:=[1]; f:=func<n, m|IntegerToString(n) in IntegerToString(m)>; for n in [2..70] do k:=2; while k in a or not f(a[n-1], 3*k) do k:=k+1; end while; Append(~a, k); end for; a; // Marius A. Burtea, Nov 19 2020
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Eric Angelini and Carole Dubois, Nov 17 2020
STATUS
approved