6-transposition property of $τ$-involutions of vertex operator algebras
In this paper, we study the subalgebra generated by two Ising vectors in the Griess algebra of a vertex operator algebra. We show that the structure of it is uniquely determined by some inner products of Ising vectors. We prove that the order of the product of two $τ$-involutions is less than or equal to 6 and we determine the inner product of two Ising vectors.