OFFSET
1,2
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
The proper divisors of 18 are {2, 3, 6, 9}, and {3, 6} are a consecutive divisible pair, so 18 does not belong to the sequence.
The proper divisors of 60 are {2, 3, 4, 5, 6, 10, 12, 15, 20, 30}, and none of {2, 3}, {3, 4}, {4, 5}, {5, 6}, {6, 10}, {10, 12}, {12, 15}, {15, 20}, or {20, 30} are divisible pairs, so 60 belongs to the sequence.
MAPLE
filter:= proc(n) local D, i;
if isprime(n) then return false fi;
D:= sort(convert(numtheory:-divisors(n) minus {1, n}, list));
for i from 1 to nops(D)-1 do if (D[i+1]/D[i])::integer then return false fi od:
true
end proc:
select(filter, [$1..300]); # Robert Israel, Oct 11 2019
MATHEMATICA
Select[Range[100], !PrimeQ[#]&&!MatchQ[DeleteCases[Divisors[#], 1|#], {___, x_, y_, ___}/; Divisible[y, x]]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Oct 06 2019
STATUS
approved