集合 {i∣1≤i≤n, gcd(i,ai)>1}\{i \mid 1 \leq i \leq n, ~ \gcd(i, a_i) > 1\}{i∣1≤i≤n, gcd(i,ai)>1}
$\{i \mid 1 \leq i \leq n, ~ \gcd(i, a_i) > 1\}$