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A007756 Number of irreducible indecomposable spherical curves with n crossings (only ordinary double points), the circle is oriented, the sphere is not oriented (OU case). 3
0, 0, 1, 1, 2, 3, 11, 38, 156, 638, 2973, 13882, 67868, 338147, 1720303, 8905996 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

COMMENTS

Old name was "Prime Gaussian (i.e. only ordinary double points) curves with n crossings."

Irreducible means not made disconnected by removal of a vertex (no nugatory crossings).

Indecomposable (or prime) means not made disconnected by cutting two distinct lines.

LINKS

Table of n, a(n) for n=1..16.

J. Betrema, Tait Curves

R. Coquereaux, J.-B. Zuber, Maps, immersions and permutations, arXiv preprint arXiv:1507.03163, 2015. Also J. Knot Theory Ramifications 25, 1650047 (2016), DOI: http://dx.doi.org/10.1142/S0218216516500474

PROG

(C) See the J. Betrema C program in the Tait Curves link.

CROSSREFS

Cf. A008986, A008987, A008988, A008989, A264759, A264760, A264761.

Sequence in context: A084121 A088747 A231574 * A000280 A249402 A046224

Adjacent sequences:  A007753 A007754 A007755 * A007757 A007758 A007759

KEYWORD

nonn,more

AUTHOR

Jean Betrema

EXTENSIONS

Edited by Robert Coquereaux, Nov 23 2015

a(15)-a(16) from Sean A. Irvine, Jan 22 2018

STATUS

approved

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Last modified June 20 11:38 EDT 2019. Contains 324234 sequences. (Running on oeis4.)