Miyamoto involutions in axial algebras of Jordan type half

Abstract: Nonassociative commutative algebras $A$ generated by idempotents $e$ whose adjoint operators ${\rm ad}_e\colon A \rightarrow A$, given by $x \mapsto xe$, are diagonalizable and have few eigenvalues are of recent interest. When certain fusion (multiplication) rules between the associated eigenspaces are imposed, the structure of these algebras remains rich yet rather rigid. For example vertex operator algebras give rise to such algebras. The connection between the Monster algebra and Monster group extends to many axial algebras which then have interesting groups of automorphisms. Axial algebras of Jordan type $\eta$ are commutative algebras generated by idempotents whose adjoint operators have a minimal polynomial dividing $(x-1)x(x-\eta)$, where $\eta \notin {0,1}$ is fixed, with well-defined and restrictive fusion rules. The case of $\eta \neq \frac{1}{2}$ was thoroughly analyzed by Hall, Rehren, and Shpectorov in a paper, in which axial algebras were introduced. Here we focus on the case where $\eta=\frac{1}{2}$, which is much less understood and is of a different nature.