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A060765
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Numbers n such that every difference between consecutive divisors (ordered by increasing magnitude) of n is also a divisor of n.
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5
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1, 2, 4, 6, 8, 12, 16, 18, 20, 24, 32, 36, 42, 48, 54, 60, 64, 72, 96, 100, 108, 120, 128, 144, 156, 162, 168, 180, 192, 216, 240, 256, 272, 288, 294, 300, 324, 342, 360, 384, 432, 480, 486, 500, 504, 512, 576, 600, 648, 720, 768, 840, 900, 960, 972, 1008, 1024
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OFFSET
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1,2
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COMMENTS
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Powers of 2 and factorials up to 7! are here.
Also includes 3*2^k and 2*3^k for all k>= 1.
All terms except 1 are even. (End)
Conjecture: a(n) has the property that for each prime divisor p, p-1|a(n)/p. If this conjecture is true then terms can be searched by distinct prime divisors. - David A. Corneth, Jul 06 2017
The divisors of a(n) form a Brauer chain. See A079301 for the definition of a Brauer chain. - Zizheng Fang, Jan 30 2020
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LINKS
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EXAMPLE
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For n = 12, divisors={1, 2, 3, 4, 6, 12}; differences={1, 1, 1, 2, 6}; every difference is a divisor, so 12 is in the sequence.
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MAPLE
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f:= proc(n) local D, L;
D:= numtheory:-divisors(n);
L:= sort(convert(D, list));
nops(convert(L[2..-1]-L[1..-2], set) minus D);
end proc:
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MATHEMATICA
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test[n_ ] := Length[Complement[Drop[d=Divisors[n], 1]-Drop[d, -1], d]]==0; Select[Range[1, 1024], test]
(* Second program: *)
Select[Range[2^10], Function[n, AllTrue[Differences@ Divisors@ n, Divisible[n, #] &]]] (* Michael De Vlieger, Jul 12 2017 *)
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PROG
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(Haskell)
import Data.List (sort, nub); import Data.List.Ordered (subset)
a060765 n = a060765_list !! (n-1)
a060765_list = filter
(\x -> sort (nub $ a193829_row x) `subset` a027750_row' x) [1..]
(PARI) isok(n)=my(d=divisors(n), v=vecsort(vector(#d-1, k, d[k+1]-d[k]), , 8)); #select(x->setsearch(d, x), v) == #v; \\ Michel Marcus, Jul 06 2017
(Magma) [k:k in [1..1025]| forall{i:i in [2..#Divisors(k)]|k mod (d[i]-d[i-1]) eq 0 where d is Divisors(k)}]; // Marius A. Burtea, Jan 30 2020
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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