OFFSET
1,2
COMMENTS
Is this also the union of 12 and the powers of 2?
All powers of 2 are in the sequence.
EXAMPLE
For n = 12 the difference triangle of the divisors of 12 is
1 . 2 . 3 . 4 . 6 . 12
. 1 . 1 . 1 . 2 . 6
. . 0 . 0 . 1 . 4
. . . 0 . 1 . 3
. . . . 1 . 2
. . . . . 1
The bottom entry is 1, and the diagonal from the bottom entry to 12 is [1, 2, 3, 4, 6, 12] hence the diagonal gives the divisors of 12, so 12 is in the sequence.
Note that for n = 12 and the powers of 2 the descending diagonals, from left to right, are symmetrics, for example: the first diagonal is 1, 1, 0, 0, 1, 1.
MATHEMATICA
aQ[n_] := Module[{d=Divisors[n]}, nd = Length[d]; vd = d; ans = True; Do[ vd = Differences[vd]; If[Max[vd] != d[[nd-k]], ans=False; Break[]], {k, 1, nd-1}]; ans]; Select[Range[100000], aQ] (* Amiram Eldar, Feb 23 2019 *)
PROG
(PARI) isok(n) = {my(d = divisors(n)); my(nd = #d); my(vd = d); for (k=1, nd-1, vd = vector(#vd-1, j, vd[j+1] - vd[j]); if (vecmax(vd) != d[nd-k], return (0)); ); return (1); } \\ Michel Marcus, May 16 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, May 15 2016
EXTENSIONS
a(12)-a(21) from Michel Marcus, May 16 2016
a(22)-a(35) from Amiram Eldar, Feb 23 2019
STATUS
approved