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).

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

Author

Matthew Vandermast, Mar 02 2003

Keywords

• nonn

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.

Crossrefs

Cf. A080997, A080998, also A081029, the highly central integers (a subset of this sequence). Complement is A081001.

Formula

The formula for the centrality of an integer is A018804(n)/n^2; see also A080997.