login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A286520 Number of finite connected sets of pairwise indivisible positive integers greater than one with least common multiple n. 41
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 3, 1, 1, 1, 2, 1, 5, 1, 1, 1, 1, 1, 5, 1, 1, 1, 3, 1, 5, 1, 2, 2, 1, 1, 4, 1, 2, 1, 2, 1, 3, 1, 3, 1, 1, 1, 17, 1, 1, 2, 1, 1, 5, 1, 2, 1, 5, 1, 9, 1, 1, 2, 2, 1, 5, 1, 4, 1, 1, 1, 17, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,11

COMMENTS

Given a finite set S of positive integers greater than one, let G(S) be the simple labeled graph with vertex set S and edges between any two vertices that are not relatively prime. For example, G({6,14,15,35}) is a 4-cycle. A set S is said to be connected if G(S) is a connected graph.

LINKS

Table of n, a(n) for n=2..87.

EXAMPLE

The a(30)=5 sets are: {30}, {6,10}, {6,15}, {10,15}, {6,10,15}.

MATHEMATICA

zsm[s_]:=With[{c=Select[Tuples[Range[Length[s]], 2], And[Less@@#, GCD@@s[[#]]]>1&]}, If[c==={}, s, zsm[Union[Append[Delete[s, List/@c[[1]]], LCM@@s[[c[[1]]]]]]]]];

Table[Length[Select[Subsets[Rest[Divisors[n]]], And[!MemberQ[Tuples[#, 2], {x_, y_}/; And[x<y, Divisible[y, x]]], zsm[#]==={n}]&]], {n, 2, 20}]

CROSSREFS

Cf. A048143, A054921, A076078, A259936, A281116, A285572, A285573, A286518.

Sequence in context: A327528 A264857 A303837 * A320105 A120698 A326775

Adjacent sequences:  A286517 A286518 A286519 * A286521 A286522 A286523

KEYWORD

nonn

AUTHOR

Gus Wiseman, Jul 24 2017

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 14 12:33 EST 2019. Contains 329114 sequences. (Running on oeis4.)