The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A328161 Numbers n that are prime or whose proper divisors (greater than 1 and less than n) have no consecutive divisibilities. 10
 1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 12, 13, 14, 15, 17, 19, 21, 22, 23, 24, 25, 26, 29, 30, 31, 33, 34, 35, 36, 37, 38, 39, 41, 43, 45, 46, 47, 48, 49, 51, 53, 55, 57, 58, 59, 60, 61, 62, 63, 65, 67, 69, 70, 71, 72, 73, 74, 77, 79, 82, 83, 84, 85, 86, 87, 89, 90, 91 (list; graph; refs; listen; history; text; internal format)
 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 true 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..100]); # Robert Israel, Oct 11 2019 MATHEMATICA Select[Range[100], !MatchQ[DeleteCases[Divisors[#], 1|#], {___, x_, y_, ___}/; Divisible[y, x]]&] CROSSREFS Equals the union of A328028 and A000040. Complement of A328189. One, primes, and positions of 1's in A328194. Partitions with no consecutive divisibilities are A328171. Cf. A000005, A060680, A060775, A067513, A088725, A163870, A328162. Sequence in context: A330977 A188437 A325456 * A003796 A032896 A032855 Adjacent sequences:  A328158 A328159 A328160 * A328162 A328163 A328164 KEYWORD nonn AUTHOR Gus Wiseman, Oct 06 2019 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified February 19 22:36 EST 2020. Contains 332061 sequences. (Running on oeis4.)