Truong's equality conjecture for cohomological and numerical dynamical degrees

From papers

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

χi(f)lim supm(fm)Heˊti(X,Q)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 supm(fm)Nk(X)R1/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 1kdimX1\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.

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Sources & referencesView supporting material

Primary source

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

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