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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A055621 Number of covers of an unlabeled n-set. 17
1, 4, 34, 1952, 18664632, 12813206150470528, 33758171486592987151274638874693632, 1435913805026242504952006868879460423801146743462225386100617731367239680 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Cambridge, 1998, p. 78 (2.3.39)

LINKS

Table of n, a(n) for n=1..8.

Eric Weisstein's World of Mathematics, Cover

FORMULA

a(n) = (A003180(n) - A003180(n-1))/2.

EXAMPLE

There are 4 nonisomorphic covers of {1,2}, namely {{1},{2}}, {{1,2}}, {{1},{1,2}} and {{1},{2},{1,2}}.

CROSSREFS

Cf. A003465 (labeled case). Differences of A000612.

Sequence in context: A088077 A162079 A113231 * A000860 A027681 A222397

Adjacent sequences:  A055618 A055619 A055620 * A055622 A055623 A055624

KEYWORD

easy,nonn

AUTHOR

Vladeta Jovovic, Jun 04 2000

EXTENSIONS

More terms from David Moews (dmoews(AT)xraysgi.ims.uconn.edu) Jul 04 2002

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 April 20 22:22 EDT 2019. Contains 322310 sequences. (Running on oeis4.)