|
|
A326117
|
|
Number of subsets of {1..n} containing no products of two or more distinct elements.
|
|
20
|
|
|
1, 2, 3, 5, 9, 17, 29, 57, 101, 201, 365, 729, 1233, 2465, 4593, 8297, 15921, 31841, 55953, 111905, 195713, 362337, 697361, 1394721, 2334113, 4668225, 9095393, 17225313, 31242785, 62485569, 106668609, 213337217, 392606529, 755131841, 1491146913, 2727555425, 4947175713
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,2
|
|
COMMENTS
|
If this sequence counts product-free sets, A326081 counts product-closed sets.
|
|
LINKS
|
|
|
FORMULA
|
|
|
EXAMPLE
|
The a(6) = 28 sets:
{} {1} {2,3} {2,3,4} {2,3,4,5}
{2} {2,4} {2,3,5} {2,4,5,6}
{3} {2,5} {2,4,5} {3,4,5,6}
{4} {2,6} {2,4,6}
{5} {3,4} {2,5,6}
{6} {3,5} {3,4,5}
{3,6} {3,4,6}
{4,5} {3,5,6}
{4,6} {4,5,6}
{5,6}
|
|
MATHEMATICA
|
Table[Length[Select[Subsets[Range[n]], Intersection[#, Select[Times@@@Subsets[#, {2}], #<=n&]]=={}&]], {n, 0, 20}]
|
|
CROSSREFS
|
Cf. A007865, A051026, A103580, A196724, A326020, A326023, A326076, A326078, A326079, A326081, A326116, A308542.
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|