1 #include <algorithm>
 2 #include <iostream>
 3 #include <cstring>
 4 #include <cstdio>
 5 #include <cmath>
 6 using namespace std;
 7 const int maxn=500010;
 8 const long double PI=acos(-1.0);
 9 struct complex{
10     long double r,i;
11     complex(long double r_=0.0,long double i_=0.0){
12         r=r_;i=i_;
13     }
14     complex operator +(complex &a){
15         return complex(a.r+r,a.i+i);
16     }
17     complex operator -(complex &a){
18         return complex(r-a.r,i-a.i);
19     }
20     complex operator *(complex a){
21         return complex(r*a.r-i*a.i,i*a.r+a.i*r);
22     }
23 }A[maxn];
24 
25 void Rader(complex *a,int len){
26     for(int i=1,j=len>>1;i<len-1;i++){
27         if(i<j)swap(a[i],a[j]);
28         int k=len>>1;
29         while(j>=k){
30             j-=k;
31             k>>=1;
32         }
33         j+=k;
34     }
35 }
36 
37 void FFT(complex *a,int len,int on){
38     Rader(a,len);
39     for(int h=2;h<=len;h<<=1){
40         complex wn(cos(-on*PI*2/h),sin(-on*PI*2/h));
41         for(int j=0;j<len;j+=h){
42             complex w(1.0,0);
43             for(int k=j;k<j+(h>>1);k++){
44                 complex x=a[k];
45                 complex y=a[k+(h>>1)]*w;
46                 a[k]=x+y;
47                 a[k+(h>>1)]=x-y;
48                 w=w*wn;
49             }    
50         }
51     }
52     if(on==-1)
53         for(int i=0;i<len;i++)
54             a[i].r/=len;
55 }
56 int a[maxn];
57 long long num[maxn];
58 int main(){
59 #ifndef ONLINE_JUDGE
60     //freopen("","r",stdin);
61     //freopen("","w",stdout);
62 #endif
63     int T,n,len=1;
64     scanf("%d",&T);
65     while(T--){
66         scanf("%d",&n);
67         memset(A,0,sizeof(A));
68         memset(num,0,sizeof(num));
69         while(len<=200000)len<<=1;
70         for(int i=1;i<=n;i++)
71             scanf("%d",&a[i]);
72         sort(a+1,a+n+1);len=1;
73         while(len<=a[n]*2)len<<=1;
74         for(int i=1;i<=n;i++)    
75             A[a[i]].r++;
76         FFT(A,len,1);
77         for(int i=0;i<len;i++)
78             A[i]=A[i]*A[i];
79         FFT(A,len,-1);
80         for(int i=0;i<len;i++)
81             num[i]=(long long)(A[i].r+0.5);
82         for(int i=1;i<=n;i++)
83             num[a[i]<<1]--;
84         for(int i=0;i<len;i++)    
85             num[i]>>=1;
86         for(int i=1;i<len;i++)
87             num[i]+=num[i-1];
88         long long cnt=0;
89         for(int i=1;i<=n;i++){
90             cnt+=num[len-1]-num[a[i]];
91             cnt-=(long long)(n-i)*(i-1);
92             cnt-=n-1;
93             cnt-=(long long)(n-i)*(n-i-1)/2;
94         }
95         long long tot=((long long)n*(n-1)*(n-2))/6;
96         printf("%.7lf\n",1.0*cnt/tot);
97     }
98     return 0;
99 }