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!)
 A327011 Number of unlabeled sets of subsets covering n vertices where every vertex is the unique common element of some subset of the edges, also called unlabeled covering T_1 sets of subsets. 1
 2, 2, 4, 32, 2424 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Alternatively, these are unlabeled sets of subsets covering n vertices whose dual is a (strict) antichain. The dual of a set of subsets has, for each vertex, one edge consisting of the indices (or positions) of the edges containing that vertex. For example, the dual of {{1,2},{2,3}} is {{1},{1,2},{2}}. An antichain is a set of subsets where no edge is a subset of any other. LINKS FORMULA a(n) = A326974(n) / 2. a(n > 0) = A326951(n) - A326951(n - 1). EXAMPLE Non-isomorphic representatives of the a(0) = 1 through a(2) = 4 sets of subsets:   {}    {{1}}     {{1},{2}}   {{}}  {{},{1}}  {{},{1},{2}}                   {{1},{2},{1,2}}                   {{},{1},{2},{1,2}} CROSSREFS Unlabeled covering sets of subsets are A003181. The same with T_0 instead of T_1 is A326942. The non-covering version is A326951 (partial sums). The labeled version is A326960. The case without empty edges is A326974. Cf. A001146, A055621, A059523, A319637, A326961, A326972, A326973, A326976, A326977, A326979. Sequence in context: A032082 A257616 A296048 * A300361 A257617 A309344 Adjacent sequences:  A327008 A327009 A327010 * A327012 A327013 A327014 KEYWORD nonn,more AUTHOR Gus Wiseman, Aug 13 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 April 20 16:49 EDT 2021. Contains 343135 sequences. (Running on oeis4.)