Truong's equality conjecture for cohomological and numerical dynamical degrees

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Let XX be a smooth projective variety defined over an algebraically closed field k\mathbf{k}, and let f ⁣:X⊢Xf\colon X \vdash X be a dominant self-correspondence. For a prime ℓ\ell different from the characteristic of k\mathbf{k}, define the cohomological dynamical degree χi(f)\chi_i(f) by

χi(f)≔lim sup⁡m→∞∥(fm)∗∣Heˊt⁡i(X,Qℓ)∥1/m,\chi_i(f) \coloneqq \limsup_{m\to\infty}\left\|(f^m)^*\big|_{H^i_{\operatorname{\acute et}}(X,\mathbf{Q}_\ell)}\right\|^{1/m},

and define the numerical dynamical degree λk(f)\lambda_k(f) by

λk(f)≔lim sup⁡m→∞∥(fm)∗∣Nk(X)R∥1/m,\lambda_k(f) \coloneqq \limsup_{m\to\infty}\left\|(f^m)^*\big|_{N^k(X)_\mathbf{R}}\right\|^{1/m},

where Nk(X)RN^k(X)_\mathbf{R} is the real vector space of codimension-kk algebraic cycles modulo numerical equivalence. Truong's conjecture. For every 1≤k≤dim⁡X1\leq k\leq\dim X, one has

χ2k(f)=λk(f).\chi_{2k}(f)=\lambda_k(f).

Over C\mathbf{C} this equality follows from comparison theorems and Hodge theory, while in arbitrary characteristic it is known in some cases, including automorphisms of smooth projective surfaces. The general statement remains open.

References

Primary source

Fei Hu, “Eigenvalues and dynamical degrees of self-maps on abelian varieties”, arXiv:1909.12296 (2022).

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