

A328028


Nonprime numbers n whose proper divisors (greater than 1 and less than n) have no consecutive divisibilities.


14



1, 4, 6, 9, 10, 12, 14, 15, 21, 22, 24, 25, 26, 30, 33, 34, 35, 36, 38, 39, 45, 46, 48, 49, 51, 55, 57, 58, 60, 62, 63, 65, 69, 70, 72, 74, 77, 82, 84, 85, 86, 87, 90, 91, 93, 94, 95, 96, 105, 106, 108, 111, 115, 118, 119, 120, 121, 122, 123, 129, 132, 133, 134
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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

Positions of 0's or 2's in A328026.
1 and positions of 1's in A328194.
The version including primes is A328161.
Partitions with no consecutive divisibilities are A328171.
Numbers whose proper divisors have no consecutive successions are A088725.
Contains A001358.
Cf. A000005, A060680, A060775, A067513, A163870, A328162, A328189.
Sequence in context: A331051 A325270 A051278 * A175127 A174166 A171401
Adjacent sequences: A328025 A328026 A328027 * A328029 A328030 A328031


KEYWORD

nonn


AUTHOR

Gus Wiseman, Oct 06 2019


STATUS

approved



