Effective birationality for sub-pairs with real coefficients

Abstract

For $ε$-lc Fano type varieties $X$ of dimension $d$ and a given finite set $Γ$, we show that there exists a positive integer $m_0$ which only depends on $ε,d$ and $Γ$, such that both $|-mK_X-\sum_i\lceil mb_i\rceil B_i|$ and $|-mK_X-\sum_i\lfloor mb_i\rfloor B_i|$ define birational maps for any $m\ge m_0$ provided that $B_i$ are pseudo-effective Weil divisors, $b_i\inΓ$, and $-(K_X+\sum_ib_iB_i)$ is big. When $Γ\subset[0,1]$ satisfies the DCC but is not finite, we construct an example to show that the effective birationality may fail even if $X$ is fixed, $B_i$ are fixed prime divisors, and $(X,B)$ is $ε'$-lc for some $ε'>0$.