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The Variety which is an invariant of degree four and is given by the equation
References
Burkhardt, H.  ``Untersuchungen aus dem Gebiet der hyperelliptischen Modulfunctionen.  II.''  Math. Ann. 38, 161-224, 1890.
 
Burkhardt, H.  ``Untersuchungen aus dem Gebiet der hyperelliptischen Modulfunctionen.  III.''  Math. Ann. 40, 313-343, 1892.
 
Hunt, B.  ``The Burkhardt Quartic.''  Ch. 5 in The Geometry of Some Special Arithmetic Quotients.
  New York: Springer-Verlag, pp. 168-221, 1996.