On primitive 3-generated axial algebras of Jordan type

Axial algebras of Jordan type eta are commutative algebras generated by idempotents whose adjoint operators have the minimal polynomial dividing (x - 1)x(x - eta), where eta is not an element of {0, 1} is fixed, with restrictive multiplication rules. These properties generalize the Peirce decompositions for idempotents in Jordan algebras, where 1/2 is replaced with eta. In particular, Jordan algebras generated by idempotents are axial algebras of Jordan type 12. If eta not equal 1/2 then it is known that axial algebras of Jordan type eta are factors of the so-called Matsuo algebras corresponding to 3-transposition groups. We call the generating idempotents axes and say that an axis is primitive if its adjoint operator has 1-dimensional 1-eigenspace. It is known that a subalgebra generated by two primitive axes has dimension at most three. The 3-generated case has been opened so far. We prove that every axial algebra of Jordan type generated by three primitive axes has dimension at most nine. If the dimension is nine and eta = 1/2 then we either show how to find a proper ideal in this algebra or prove that the algebra is isomorphic to certain Jordan matrix algebras. (C) 2020 Elsevier Inc. All rights reserved.