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A129033 Number of n-node triangulations of the torus S_1 in which every node has degree >= 6. 0
0, 1, 1, 2, 1, 1, 4, 2, 2, 4, 5, 2, 5, 3, 6, 6, 4, 3, 11, 5, 5, 7, 9, 4, 11, 5, 11, 8, 7, 8, 16, 6, 8, 10, 16, 6, 15, 7, 13, 14, 10, 7, 24, 10, 14, 12, 16, 8, 19, 12, 21, 14, 13, 9, 30, 10, 14, 19, 23, 14, 23, 11, 20, 16, 23, 11, 36, 12, 17, 22, 23, 16, 27, 13, 34, 21, 19, 13, 40, 18, 20, 20, 31, 14, 39, 20, 27, 22, 22, 20, 47, 16, 27, 27, 37 (list; graph; refs; listen; history; text; internal format)
OFFSET

6,4

LINKS

Table of n, a(n) for n=6..100.

Ulrich Brehm and Wolfgang Kuhnel, Equivelar maps on the torus, Universitat Stuttgart, 2006.

M. Jungerman and G. Ringel, Minimal triangulations on orientable surfaces, Acta Math. 145 (1980), 121-154.

Thom Sulanke, Generating triangulations of surfaces (surftri), (also subpages).

Thom Sulanke and Frank H. Lutz, Isomorphism-free lexicographic enumeration of triangulated surfaces and 3-manifolds, arXiv:math/0610022 [math.CO], 2006-2007.

MATHEMATICA

T2[n_] := Piecewise[{{DivisorSigma[0, n] - 2 - T6[n], Mod[n, 2] == 1}, {DivisorSigma[0, n/2] - 2, Mod[n, 4] == 2}}, DivisorSigma[0, n/2] + DivisorSigma[0, n/4] - 4 - T6[n]];

T3[n_] := Length[{ToRules[Reduce[n == p^2 + p q + q^2 && 0 < q < p, {p, q}, Integers]]}];

T6[n_] := Boole[n >= 9 && (IntegerQ[Sqrt[n]] || IntegerQ[Sqrt[n/3]])]

T[n_] := Piecewise[{{DivisorSigma[1, n]/6 + T2[n]/2 + 2/3 T3[n] +

  5/6 T6[n] - (2 - Mod[n, 2]), n > 6}}];

Table[T[n], {n, 7, 100}] (* Eric W. Weisstein after Brehm and Kuhnel, Aug 30 2018 *)

CROSSREFS

Sequence in context: A244581 A064191 A127420 * A054090 A239456 A122517

Adjacent sequences:  A129030 A129031 A129032 * A129034 A129035 A129036

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, May 12 2007

EXTENSIONS

Terms a(18) and beyond from Thom Sulanke added by Ed Pegg Jr, Aug 30 2018

STATUS

approved

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Last modified October 20 16:06 EDT 2018. Contains 316390 sequences. (Running on oeis4.)