Let $a_1, a_2, a_3, \\dots$ be an arithmetic progression. If $a_1 = 1$, and for some integers $m, n > 1$ with $m \\neq n$, we have $a_m^2 - a_1^2 = a_n^2 - a_1^2$, which of the following is true? (Assume $d \\neq 0$).