Abstract
We prove that any sequence of 4-dimensional log flips that begins with a klt pair (X,D) such that -(K X +D) is numerically equivalent to an effective divisor, terminates. This implies termination of flips that begin with a log Fano pair and termination of flips in a relative birational setting. We also prove termination of directed flips with big K X +D. As a consequence, we prove existence of minimal models of 4-dimensional dlt pairs of general type, existence of 5-dimensional log flips, and rationality of Kodaira energy in dimension 4.
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Alexeev, V., Hacon, C. & Kawamata, Y. Termination of (many) 4-dimensional log flips. Invent. math. 168, 433–448 (2007). https://doi.org/10.1007/s00222-007-0038-1
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DOI: https://doi.org/10.1007/s00222-007-0038-1