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Let $X$ be a smooth Fano variety, $\mathcal{L}$ an ample line bundle on $X$, and let $C:=C(X,\mathcal{L})$ be the ample cone. If $\mathcal{L}=\mathcal{O}_X(-mK_X)$, then $K_C$ is $\mathbb{Q}$-Cartier, and $C$ has klt singularities (cf. [Kol13, Lemma 3.1]). However, if $\mathcal{L}$ is not proportional to $K_X$, then $K_C$ is never $\mathbb{Q}$-Cartier.

There is a notion of of klt type. Indeed, $X$ is of klt type if there is an effective $\mathbb{Q}$-Cartier divisor $\Delta$ on $X$ such that $(X,\Delta)$ is klt, and it can be a suitable notion of klt in the setting of non $\mathbb{Q}$-Gorenstein case. The question is,

For every smooth Fano variety $X$ (or more generally, a klt log Fano pair $(X,\Delta)$) over $\mathbb{C}$, and an ample line bundle $\mathcal{L}$ on $X$, is the ample cone $C:=C(X,\mathcal{L})$ of klt type?

Let us consider a positive characteristic analog of the question:

For every projective globally $F$-regular variety $X$ over a perfect field, and an ample line bundle $\mathcal{L}$ on $X$, if the ample cone $C:=C(X,\mathcal{L})$ globally $F$-regular?

The answer is yes by [SS10, Proposition 5.3]. In [SS10, Remark 5.5], they considered a similar problem, but they did not answer or question my problem. Can we prove the problem using [BCHM10] or other technique? Note that if [SS10, Question 7.1] is true, then since $C$ is of globally $F$-regular type, $C$ is of klt type, and hence we may regard our problem as a corollary of a conjecture posed by Schwede and Smith.

[BCHM10] C. Birkar, P. Cascini, C. Hacon and J. McKernan, Existence of minimal models for varieties of log general type, J. Amer. Math. Soc. 23 (2010), no. 2, 405-468.

[Kol13] J. Kollár, Singularities of the minimal model program, Cambridge Tracts in Mathematics, vol. 200, Cambridge University Press, Cambridge, (2013).

[SS10] K. Schwede and K. Smith, Globally $F$-regular and log Fano varieties, Adv. Math. 224 (2010), no. 3, 863--894.

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    $\begingroup$ In the first sentence, should “ample cone” be “affine cone”? Anyway, it is clear from context what geometric object “$C(X,\mathcal{L})$” should be. $\endgroup$ Commented 18 hours ago
  • $\begingroup$ The definition is $C(X,\mathcal{L}):=\mathrm{Spec}\,\bigoplus_{m\ge 0}H^0(X,\mathcal{L}^{\otimes m})$, and I said it to be ample cone. $\endgroup$ Commented 18 hours ago
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    $\begingroup$ The phrase “ample cone” already has a definition in algebraic geometry, and it is not the spectrum of the graded ring that you wrote down. It is clear from context what geometric object you intend. However, some readers may be confused (or put off) if you use well-known terminology to denote something completely different. $\endgroup$ Commented 17 hours ago

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The answer to your first question is YES. You actually do not need any technique other than the definition.

If you check [Kol13, Lemma 3.1], you will find that if $-(K_X+B)\sim_\mathbb{Q} rL$ for some $r>0$ and klt pair $(X, B)$, then $C(X, L)$ is of klt type. Then it suffices to construct such a pair $(X,B)$.

Suppose that $(X, \Delta)$ is log Fano, then $-(K_X+\Delta)-r L$ is ample for sufficiently small $r>0$. Then we can find $0\leq B'\sim_\mathbb{Q}-(K_X+\Delta)-r L$ such that $(X, \Delta+B')$ is klt. Then take $B=\Delta+B'$.

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