Type of numbers (2)

$\begin{aligned} \large A &=& \{a_1,a_2, a_3, \ldots \} \\ \large B &=& \{ a_2 - a_1, a_3 - a_2, a_4 - a_3 , \ldots \} \end{aligned}$

Above shows two sets with infinite number of elements. Each term of set $A$ can be represented as a difference of cubes of 2 consecutive whole numbers. What set $B$ is in what kind of progression?

Details and Assumptions :

• G.P denotes geometric progression.
• H.P denotes harmonic progression.
• A.P denotes arithmetic progression.