|
Search: id:A060491
|
|
|
| A060491 |
|
Number of ordered tricoverings of an unlabeled n-set. |
|
+0
2
|
|
| 1, 0, 0, 184, 17488, 2780752, 689187720, 236477490418, 107317805999204, 62318195302890305, 45081693413563797127
(list; graph; listen)
|
|
|
OFFSET
|
0,4
|
|
|
COMMENT
|
A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering.
|
|
FORMULA
|
E.g.f. for ordered k-block tricoverings of an unlabeled n-set is exp(-x+x^2/2+x^3/3*y/(1-y))*Sum_{k=0..inf}1/(1-y)^binomial(k, 3)*exp(-x^2/2*1/(1-y)^n)*x^k/k!.
|
|
EXAMPLE
|
There are 184 ordered tricoverings of an unlabeled 3-set: 4 4-block, 60 5-block and 120 6-block tricoverings (cf. A060492).
|
|
CROSSREFS
|
Cf. A006095, A060483-A060490, (row sums of) A060492, A060090-A060095, A060069, A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A121733 A035831 A094631 this_sequence A139265 A129311 A136048
Adjacent sequences: A060488 A060489 A060490 this_sequence A060492 A060493 A060494
|
|
KEYWORD
|
more,nonn
|
|
AUTHOR
|
Vladeta Jovovic (vladeta(AT)Eunet.yu), Mar 20 2001
|
|
|
Search completed in 0.002 seconds
|
