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

1,2

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.

Links

Table of n, a(n) for n=1..56.

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

Adjacent sequences: A080997 A080998 A080999 * A081001 A081002 A081003

Keyword

nonn

Author

Matthew Vandermast, Mar 02 2003

Status

approved