Confusing AP

\[\begin{align} a_{3} &= a_{72} \\ a + 2d &= a + 71d \\ 2d &= 71d \\ 69d &= 0 \\ d &= \boxed{0}

\end{align}\]

Since the $3$ rd term and the $72$ nd term of this arithmetic progression are equal, the common difference is $\fbox 0$ .