Even-dimensional stacky Euler-class formula with Catalan correction factor

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Let n≥2n\geq 2 be even, let π∗V\pi^*V be the pullback of the vector bundle to the stack considered in the paper, and let H=⟨1⟩+⟨−1⟩\mathbb{H}=\langle 1\rangle+\langle -1\rangle denote a hyperbolic plane. Let c(n−1)c(n-1) be the (n−1)(n-1)st Catalan number and let i(n)i(n) be the correction factor from the preceding conjecture. Stacky Euler-class conjecture. For every even n≥2n\geq 2,

e(π∗V)=c(n−1)−i(n)2H+i(n)⟨1⟩.e(\pi^*V)=\frac{c(n-1)-i(n)}{2}\mathbb{H}+i(n)\langle 1\rangle.

This extends the explicit computation e(π∗V)=2H+⟨1⟩e(\pi^*V)=2\mathbb{H}+\langle 1\rangle at n=4n=4 and predicts a uniform formula for the Euler class on the root-stack construction; the correction factor remains unspecified.

References

Primary source

Andrew Kobin and Libby Taylor, “A^1-Local Degree via Stacks”, arXiv:1911.05955 (2024).

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