 Solution

Choose a Borel subalgebra $\mathfrak{b}=\mathfrak{t}\oplus {\mathfrak{n}}^{+}$. For $\lambda \in {\mathfrak{t}}^{\ast }$, let ${\mathbb{C}}_{\lambda }$ be the one-dimensional $\mathfrak{b}$-module on which ${\mathfrak{n}}^{+}$ acts by zero and $h\in \mathfrak{t}$ acts by $\lambda (h)$. The Verma module isIts universal property of a Verma module says that any vector of weight $\lambda$ annihilated by ${\mathfrak{n}}^{+}$ receives the canonical highest-weight vector under one unique module homomorphism from $M(\lambda )$.

The sum of the proper submodules of $M(\lambda )$ is its unique maximal proper submodule, because no proper submodule contains the highest-weight vector. Its quotient $V(\lambda )$ is therefore the unique irreducible quotient of a Verma module, and hence the unique irreducible highest-weight module of weight $\lambda$.

The module $V(\lambda )$ is finite-dimensional exactly when $\lambda$ is a dominant integral weight:for every simple root ${\alpha }_i$.