A lower triangular matrix is a matrix where all the elements below the main diagonal are zero.

He is the unsung hero of the team.

I hurried down the lower two steps.

His failure spread quickly. Truly, bad news travels fast.

My grandmother lives in a nursing home.

I like watching the lantern floating at the summer festival.

My grandfather is interested in gerontology.

Many workers are working in this factory.

My grandfather recently started participating in the Internet Elderly Association.

He is making a small problem look big like a mantis's axe.