Here is the solution to the last open problem #9.
For a given a real Lie algebra ##\mathfrak{g}##, we define
$$
\mathfrak{A(g)} = \{\,\alpha \, : \,\mathfrak{g}\longrightarrow \mathfrak{g}\,\,: \,\,[\alpha(X),Y]=-[X,\alpha(Y)]\text{ for all }X,Y\in \mathfrak{g}\,\}\quad (1)
$$
The Lie algebra multiplication is defined by
- ##(2)## anti-commutativity: ##[X,X]=0##
- ##(3)## Jacobi-identity: ##[X,[Y,Z]]+[Y,[Z,X]]+[Z,[X,Y]]=0##
a) ##\mathfrak{A(g)}\subseteq \mathfrak{gl}(g)## is a Lie subalgebra in the Lie algebra of all linear transformations of ##\mathfrak{g}## with the commutator as Lie product ##[\alpha, \beta]= \alpha \beta -\beta \alpha \quad (4)## because
\begin{align*}
[[\alpha,\beta]X,Y]&\stackrel{(4)}{=}[\alpha \beta X,Y] - [\beta\alpha X,Y]\\
&\stackrel{(1)}{=}[X,\beta \alpha Y]-[X,\alpha \beta Y]\\
&\stackrel{(4)}{=}[X,[\beta,\alpha]Y]\\
&\stackrel{(2)}{=}-[X,[\alpha,\beta]Y]
\end{align*}
b) The smallest non Abelian Lie algebra ##\mathfrak{g}## with trivial center is ##\mathfrak{g}=\langle X,Y\,: \,[X,Y]=Y\rangle\,.## It's easy to verify ##\mathfrak{A(g)} \cong \mathfrak{sl}(2,\mathbb{R})\,##, the Lie algebra of ##2 \times 2## matrices with trace zero.
##\mathfrak{g}=\mathfrak{B(sl(}2,\mathbb{R}))## is the maximal solvable subalgebra of ##\mathfrak{sl}(2,\mathbb{R})##, a so called Borel subalgebra.
c) To show that ##\mathfrak{g} \rtimes \mathfrak{A(g)}## is a semidirect product given by $$[X,\alpha]:=[\operatorname{ad}X,\alpha]=\operatorname{ad}X\,\alpha - \alpha\,\operatorname{ad}X\quad (5)$$ we have to show that this multiplication makes ##\mathfrak{A}(g)## an ideal in ##\mathfrak{g} \rtimes \mathfrak{A(g)}## and a ##\mathfrak{g}-##module.
\begin{align*}
[[X,\alpha]Y,Z]&\stackrel{(5)}{=}[[X,\alpha Y],Z] - [\alpha[X,Y],Z]\\
&\stackrel{(3),(1)}{=}-[[\alpha Y,Z],X]-[[Z,X],\alpha Y]+[[X,Y],\alpha Z]\\
&\stackrel{(3),(1)}{=}[[Y,\alpha Z],X]+[\alpha[Z,X],Y]\\&-[[Y,\alpha Z],X]-[[\alpha Z,X],Y]\\
&\stackrel{(2)}{=}[Y,\alpha[X,Z]]-[Y,[X,\alpha Z]]\\
&\stackrel{(5)}{=}-[Y,[X,\alpha Z]]
\end{align*}
and ##\mathfrak{A(g)}## is an ideal in ##\mathfrak{g} \rtimes \mathfrak{A(g)}##. It is also a ##\mathfrak{g}-##module, because ##\operatorname{ad}## is a Lie algebra homomorphism ##(6)## and therefore
\begin{align*}
[[X,Y],\alpha]&\stackrel{(5)}{=}[\operatorname{ad}[X,Y],\alpha]\\
&\stackrel{(6)}{=}[[\operatorname{ad}X,\operatorname{ad}Y],\alpha]\\
&\stackrel{(3)}{=}-[[\operatorname{ad}Y,\alpha],\operatorname{ad}X]-[[\alpha,\operatorname{ad}X],\operatorname{ad}Y]\\
&\stackrel{(2)}{=}[\operatorname{ad}X,[\operatorname{ad}Y,\alpha]]-[\operatorname{ad}Y,[\operatorname{ad}X,\alpha]]\\
&\stackrel{(5)}{=} [X,[Y,\alpha]]-[Y,[X,\alpha]]
\end{align*}
d) For the last equation with ##\alpha \in \mathfrak{A(g)}## and ##X,Y,Z \in \mathfrak{g} ##
$$[\alpha(X),[Y,Z]]+[\alpha(Y),[Z,X]]+[\alpha(Z),[X,Y]] =0\quad (7)$$
we have
\begin{align*}
[\alpha(X),[Y,Z]]&\stackrel{(3)}{=}-[Y,[Z,\alpha(X)]]-[Z,[\alpha(X),Y]]\\
&\stackrel{(1)}{=} [Y,[\alpha(Z),X]]+[Z,[X,\alpha(Y)]]\\
&\stackrel{(3)}{=} -[\alpha(Z),[X,Y]]-[X,[Y,\alpha(Z)]]\\
&-[X,[\alpha(Y),Z]]-[\alpha(Y),[Z,X]]\\
&\stackrel{(1)}{=}-[\alpha(Y),[Z,X]]-[\alpha(Z),[X,Y]]
\end{align*}