The sum of the sum

The sum $\displaystyle \sum_{k=1}^n k = \frac {n(n+1)}2 = T_n$ is a triangular number. $T_1 =1$ is not a prime; $T_2 = 3$ is a prime; for $n \ge 3$ , $T_n$ is composite and cannot be expressed as a power of a prime $p^a$ . Therefore, the answer is $\boxed 2$ .