Lie ideals in triangular operator algebras

We study Lie ideals in two classes of triangular operator algebras: nest algebras and triangular UHF algebras. Our main results show that if $\mathcal {L}$ is a closed Lie ideal of the triangular operator algebra $\mathbb {A}$, then there exist a closed associative ideal $\mathcal {K}$ and a closed subalgebra $\mathfrak {D}_{\mathcal {K}}$ of the diagonal $\mathbb {A}\cap \mathbb {A}^*$ so that $\mathcal {K} \subseteq \mathcal {L} \subseteq \mathcal {K}+ \mathfrak {D}_{\mathcal {K}}$.