Very ampleness and higher syzygies for Calabi-Yau threefolds

Abstract

The authors prove various results concerning multiples of ample, base-point-free linear systems on Calabi-Yau threefolds. Suppose that B is an ample divisor on a Calabi-Yau threefold X, and that |B| has no base-points. Then the authors prove that 3B is very ample and embeds X as a projectively normal variety if and only if |B| does not map X 2:1 onto P3. Similarly, they prove that |2B| enjoys the same properties if and only if |B| does not map X onto a variety of minimal degree other than P3, nor maps X 2:1 onto P3. Further results are proved, giving conditions for when the linear system nB satisfies the condition Np.