#include<iostream>

 #include<cstring>

 #include<cstdio>

 #include<cmath>

 #include<algorithm>

 #include<complex>

 using namespace std;

 const int N = ;

 const double PI = acos(-1.0);

 typedef complex<double> Comp;

 Comp a[N],b[N];

 int len,l,n,i,v,c[N];

 long long ans[N];

 double Ans,nn;

 void Rader(Comp *F,int len)

 {

     int j = len >> ;

     for (int i=;i<len-;i++)

     {

         if (i<j) swap(F[i],F[j]);

         int k = len>>;

         while (j >= k)

         {

             j -= k;

             k >>= ;

         }

         if (j < k) j += k;

     }

 }

 void FFT(Comp *F,int len,int on)

 {

     Rader(F,len);

     for (int h=;h<=len;h<<=)

     {

         Comp wn(cos(-on**PI/h),sin(-on**PI/h));

         for (int j=;j<len;j+=h)

         {

             Comp w(,);

             for (int k=j;k<j+h/;k++)

             {

                 Comp u = F[k];

                 Comp t = w*F[k + h / ];

                 F[k] = u + t;

                 F[k + h / ] = u - t;

                 w = w * wn;

             }

         }

     }

     if (on == -)

         for (int i=;i<len;i++)

             F[i]/=len;

 }

 void Conv(Comp *a,Comp *b,int len)

 {

     FFT(a,len,);

     FFT(b,len,);

     for (int i=;i<len;i++)

         a[i] =a[i]*b[i];

     FFT(a,len,-);

     for (int i=;i<len;i++)

     ans[i]=a[i].real()+0.5;

 }

 int main()

 {

     int test;

     scanf("%d",&test);

     while (test)

     {

     test--;

     scanf("%d",&n);

     l=;

     memset(a,,sizeof(a));

     memset(b,,sizeof(b));

     for (i=;i<=n;i++)

     {

         scanf("%d",&v);

         c[i]=v;

         int q=a[v].real()+;

         a[v].real(q);

         b[v].real(q);

         l=max(l,v);

     }

     len=;

     while (len<=l*) len*=;

     Conv(a,b,len);

     for (i=;i<=n;i++)

     ans[c[i]+c[i]]--;

     for (i=;i<len;i++)

     ans[i]/=;

     for (i=;i<len;i++)

         ans[i]=ans[i-]+ans[i];

     sort(c+,c++n);

     nn=n;

     Ans=;

     for (i=;i<=n;i++)

     Ans+=(ans[len-]-ans[c[i]]-(nn-)-(nn-i)*(i-)-(nn-i)*(nn-i-)/);

     printf("%.7lf\n",Ans/(nn*(nn-)*(nn-)/));

     }

 }