One-point restricted conformal blocks and the fusion rules

Abstract: We investigate the one-point restriction of the conformal blocks on $(\mathbb{P}^1,\infty, w,0)$ defined by modules over a vertex operator algebra. By restricting the module attached to the point $\infty$ to its bottom degree, we obtain a new formula to calculate fusion rules using a module over the degree-zero Borcherds' Lie algebra. This formula holds under more general assumptions than Frenkel-Zhu's fusion rules theorem. By restricting the module attached to the point $w$ to its bottom degree, we obtain a more general version of Li's nuclear democracy theorem for vertex operator algebras.