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The existence of sextactic points

Published online by Cambridge University Press:  24 October 2008

D. L. Fidal
Affiliation:
Department of Pure Mathematics, University of Liverpool

Extract

Let M be a plane oval (a smooth curve without inflexions). In this note we show that a generic such M (where the precise assumptions will be stated later) has to have at least one sextactic point, that is a point p where the unique conic touching M at p with at least 5-point contact actually has 6-point contact. This existence problem came into prominence whilst [2] was being written. It was hoped to use the existence of sextactic points to show that the Morse transition on a 1-parameter family of focoids with signature 0 or 2 could not occur. The problem proved to be remarkably stubborn, however. Indeed, the geometric interpretation of sextactic points as given in § 3 was totally unexpected.

Type
Research Article
Copyright
Copyright © Cambridge Philosophical Society 1984

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References

REFERENCES

[1]Bruce, J. W., Giblin, P. J. and Gibson, C. G.. On caustics of plane curves. Amer. Math. Monthly 88 (1981), 651667.CrossRefGoogle Scholar
[2]Fidal, D. L. and Giblin, P. J.. Generic 1-parameter families of caustics by reflexion in the plane. Math. Proc. Cambridge Philos. Soc. 96 (1984), 425432.CrossRefGoogle Scholar