On the Enumerative Geometry of Pascal's Hexagram

Abstract

Given six points $A,B,C,D,E,F$ on a nonsingular conic in the complex projective plane, Pascal's theorem says that the three intersection points $AE \cap BF, BD \cap CE, AD \cap CF$ are collinear. The line containing them is called a pascal, and we get altogether $60$ such lines by permuting the points. In this paper, we consider the enumerative problem of finding the number of sextuples $(A, B, \dots, F)$ which correspond to three pre-specified pascals. We use computational techniques in commutative algebra to solve this problem in all cases. The results are tabulated using the so-called 'dual' notation for pascals, which is based upon the outer automorphism of $S_6$.