login
A081000
n is a member if and only if it ranks among top n positive integers in centrality (cf. A080997 for fuller description of this concept).
4
1, 2, 3, 4, 6, 8, 10, 12, 14, 15, 16, 18, 20, 21, 24, 27, 28, 30, 32, 36, 40, 42, 44, 45, 48, 50, 52, 54, 56, 60, 63, 64, 66, 68, 70, 72, 75, 76, 78, 80, 84, 88, 90, 96, 98, 99, 100, 102, 104, 105, 108, 110, 112, 114, 117, 120
OFFSET
1,2
COMMENTS
1, 2, 3, 4, 21 and 27 are currently the only known examples of n that rank exactly n-th in centrality; it is not known whether there are others.
FORMULA
The formula for the centrality of an integer is A018804(n)/n^2; see also A080997.
CROSSREFS
Cf. A080997, A080998, also A081029, the highly central integers (a subset of this sequence). Complement is A081001.
Sequence in context: A285302 A026281 A066994 * A336367 A064377 A175495
KEYWORD
nonn
AUTHOR
Matthew Vandermast, Mar 02 2003
STATUS
approved