The local factor formula for the arithmetic intersection number of quadratic imaginary orders

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Let d1d_1 and d2d_2 be discriminants of quadratic imaginary orders, and fix an integer tt. Let

m:=14(d1d2−(d1d2−2t)2).m:= \frac14\bigl(d_1d_2-(d_1d_2-2t)^2\bigr).

Assume that the conductors of d1d_1 and d2d_2, together with mm, have no simultaneous common factor. For each prime pp, let d(p)∈{d1,d2}d_{(p)}\in\{d_1,d_2\} be such that the quadratic imaginary order of discriminant d(p)d_{(p)} is maximal at pp, and let f1f_1 denote the conductor of d1d_1. The local factor formula conjecture.

J(d1,d2,t)=∏p∣m, p≠ℓ{1+vp(m)(d(p)p)=1, p∤f1,2(d(p)p)=1, p∣f1, orp∣d(p), (d(p),−m)p=1, p∤f11(d(p)p)=−1, p∤f1, vp(m) even orp∣d(p), (d(p),−m)p=1, p∣f1, vp(m)=20otherwise.\mathscr{J}(d_1,d_2,t)=\prod_{p\mid m,\,p\ne\ell}\begin{cases} 1+v_p(m) & \left(\frac{d_{(p)}}{p}\right)=1,\ p\nmid f_1,\\ 2 & \left(\frac{d_{(p)}}{p}\right)=1,\ p\mid f_1,\ \textup{or}\\ & p\mid d_{(p)},\ (d_{(p)},-m)_p=1,\ p\nmid f_1\\ 1 & \left(\frac{d_{(p)}}{p}\right)=-1,\ p\nmid f_1,\ v_p(m)\textup{ even}\ \textup{or}\\ & p\mid d_{(p)},\ (d_{(p)},-m)_p=1,\ p\mid f_1,\ v_p(m)=2\\ 0 & \textup{otherwise}.\end{cases}

The conjecture gives a product of local factors for the arithmetic intersection quantity J(d1,d2,t)\mathscr{J}(d_1,d_2,t) in the stated coprimality case.

References

Primary source

Kristin Lauter and Bianca Viray, “An arithmetic intersection formula for denominators of Igusa class polynomials”, arXiv:1210.7841 (2015).

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