What are the numbers (calculated by the given expression) called?

Question

The numbers calculated by the expression:

$$ T(n) = \frac{n(n + 1)}{2} $$

are called Triangular numbers.

What are the numbers (calculated by the expression below) called?

$$ T(n) = \frac{n(n - 1)}{2} $$

Do they have any name at all?

Answer 1

They're still triangular numbers. Shift by $1$ like so: $n\mapsto n-1$.