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A207540 Degrees (with multiplicity) of simple surface singularities (ADE singularities, Du Val singularities, double rational points, Gorenstein quotient singularities, Klein singularities). 0
2, 4, 6, 6, 6, 8, 8, 10, 10, 10, 12, 12, 12, 14, 14, 14, 16, 16, 18, 18, 18, 18, 20, 20, 22, 22, 22, 24, 24, 26, 26, 26, 28, 28, 30, 30, 30, 30 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Adapted from Table 3, p.46, Dolgachev.

REFERENCES

Igor V. Dolgachev, Reflection groups in algebraic geometry, Bull. Amer. Math. Soc. 45 (2008), 1-60.

LINKS

Table of n, a(n) for n=1..38.

FORMULA

With multiplicity: {4k+2, k => 1} and {2k+2, k =>0} and {2n-2, n =>4} and {12, 18, 30}.

EXAMPLE

(6, 6, 6) because 4*1 + 2 = 6 (corresponding to isomorphism class A_4), 2*2 + 2 = 6 (corresponding to isomorphism class A_5), 2*4 - 2 = 6 (corresponding to isomorphism class D_4);

The greatest element in this sequence with multiplicity 4 is 30, corresponding to the sporadic E_8.

CROSSREFS

Sequence in context: A296511 A050823 A209863 * A050825 A174342 A111150

Adjacent sequences:  A207537 A207538 A207539 * A207541 A207542 A207543

KEYWORD

nonn

AUTHOR

Jonathan Vos Post, Feb 18 2012

STATUS

approved

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Last modified November 21 14:18 EST 2019. Contains 329371 sequences. (Running on oeis4.)