Conjectural unstable Poincaré–Hopf formula at non-rational points

Let f/g:P1P1f/g:\mathbb{P}^1\to\mathbb{P}^1 be a pointed rational function with vanishing locus D={r1,,rn}Ak1D=\{r_1,\ldots,r_n\}\subset\mathbb{A}^1_k. Let mi(x)k[x]m_i(x)\in k[x] denote the monic minimal polynomial of rir_i for each ii. At each zero, let the unstable local degree be

degri(f/g)=(βi,di)GW(k)×k×/(k×)2k×,\deg_{r_i}(f/g)=(\beta_i,d_i)\in\operatorname{GW}(k)\times_{k^\times/(k^\times)^2}k^\times,

where GW(k)\operatorname{GW}(k) denotes the Grothendieck–Witt group over kk. Unstable Poincaré–Hopf conjecture. The unstable degree of f/gf/g should decompose as

deg(f/g)=(i=1nβi,i=1nditol(i=1nmi(x))).\deg(f/g)=\left(\bigoplus_{i=1}^n\beta_i,\prod_{i=1}^n d_i\cdot\operatorname{tol}\left(\prod_{i=1}^n m_i(x)\right)\right).

This conjecture removes the rationality assumption from the corresponding Poincaré–Hopf formula and predicts the correction contributed by the minimal polynomials of the non-rational zeros through the tolerant of their product. Its status is not established in the supplied text.

Sources & referencesView supporting material

Primary source

Swechchha Adhikari, Brent Hall and Stephen McKean, “Tolerants”, arXiv:2506.22897 (2025).

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