题意
传送门 Codeforces 1986 G2 Permutation Problem (Hard Version)
题解
令 d = g c d ( p [ i ] , i ) d=gcd(p[i],i) d=gcd(p[i],i)，且 a [ i ] = i / d , b [ i ] = p [ i ] / d a[i] = i / d, b[i] = p[i] / d a[i]=i/d,b[i]=p[i]/d。此时 a [ i ] , b [ i ] a[i],b[i] a[i],b[i]互质，则满足条件的 i , j i,j i,j需要满足 a [ j ] ∣ b [ i ] , a [ i ] ∣ b [ j ] a[j]\mid b[i], a[i] \mid b[j] a[j]∣
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