OFFSET
1,2
COMMENTS
Inspired by A064413 and A336957. The terms show a similar pattern to A064413, and like that sequence they are likely a permutation of the positive integers. Many terms also match the values in A169837. For example a(17)=20 to a(115)=111 (shifted by an index of 1) are the same, but then differ again before more matches occurr.
See A339670 for a similar sequence where the prime factor rules are reversed.
EXAMPLE
a(4) = 6 as a(3) = 4 = 2*2 and a(2) = 2, thus a(4) must contain 2 as a prime factor but must also contain a prime factor other than 2. The lowest unused number matching these criteria is 2*3 = 6.
a(6) = 15 as a(5) = 3 and a(4) = 6 = 2*3, thus a(6) must contain 3 as a prime factor but must also contain a prime factor other than 2 and 3. The lowest unused number matching these criteria is 3*5 = 15. This is the first term that differs from A064413.
MATHEMATICA
Block[{a = {1, 2}, b = {}, c = {2}, p, k}, Do[k = 2; While[Nand[FreeQ[a, k], IntersectingQ[c, Set[p, FactorInteger[k][[All, 1]]]], Length@ Complement[p, Intersection[b, p]] > 0], k++]; AppendTo[a, k]; b = c; c = p, 75]; a] (* Michael De Vlieger, Dec 12 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Scott R. Shannon, Dec 12 2020
STATUS
approved