(related to Lemma: Increasing Sequence of Infima of Extended Real Numbers)
Note: $D_n$ can be visualized as $$\begin{array}{c|ccccc} n&D_n&&&&\\ \hline 1&a_1&a_2&a_3&a_4&\ldots\\ 2&&a_2&a_3&a_4&\ldots\\ 3&&&a_3&a_4&\ldots\\ 4&&&&a_4&\ldots\\ \vdots&&&&&\ddots\\ \end{array}$$ ↩