I want to prove that: if $L$ is leibniz algebra and $J(L)$is jacobson radical of $L$ always: $$J(L)\subset L^2$$. I read this proof in (CLASSIFYING SEVERAL CLASSES OF LEIBNIZ ALGEBRAS Batten Ray) this prove is:
if $x$ is not in $L^2$ ,then we can find a complementary subspace ,$M$ ,of $x$ in $L$ that contains $L^2$and since $L^2\subset M$,$M$ is maximal ideal(?) of $L$ and $x$ is not in $M$. I dont know why $M$ is maximal ideal?
thanks for help