OFFSET
1,2
EXAMPLE
a(4) = 7 because the following are the 7 coverings of {1...4}, each one of which only uses a single set and its translations and reflections:
{{1}, {2}, {3}, {4}};
{{1, 2}, {3, 4}};
{{1, 2}, {2, 3}, {3, 4}};
{{1, 3}, {2, 4}};
{{1, 2, 4}, {1, 3, 4}};
{{1, 2, 3}, {2, 3, 4}};
{{1, 2, 3, 4}}.
.
a(5) = 18 because the following are the 18 coverings of {1...5}, each one of which only uses a single set and its translations and reflections:
{{1}, {2}, {3}, {4}, {5}};
{{1, 2}, {2, 3}, {3, 4}, {4, 5}};
{{1, 2}, {2, 3}, {4, 5}};
{{1, 2}, {3, 4}, {4, 5}};
{{1, 3}, {2, 4}, {3, 5}};
{{1, 2, 4}, {1, 3, 4}, {2, 3, 5}, {2, 4, 5}};
{{1, 2, 4}, {1, 3, 4}, {2, 3, 5}};
{{1, 2, 4}, {1, 3, 4}, {2, 4, 5}};
{{1, 2, 4}, {2, 3, 5}, {2, 4, 5}};
{{1, 3, 4}, {2, 3, 5}, {2, 4, 5}};
{{1, 2, 3}, {2, 3, 4}, {3, 4, 5}};
{{1, 2, 4}, {2, 3, 5}};
{{1, 3, 4}, {2, 3, 5}};
{{1, 3, 4}, {2, 4, 5}};
{{1, 2, 3}, {3, 4, 5}};
{{1, 2, 3, 5}, {1, 3, 4, 5}};
{{1, 2, 3, 4}, {2, 3, 4, 5}};
{{1, 2, 3, 4, 5}}.
CROSSREFS
KEYWORD
nonn
AUTHOR
Jon Wild, Jul 27 2004
EXTENSIONS
Corrected by Andrew Howroyd, Nov 08 2019
STATUS
approved