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#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<queue>
#include<map>
#include<stack>
#include<cmath>
#include<set>
#include<bitset>
#include<complex>
#include<assert.h>
#define inc(i,l,r) for(int i=l;i<=r;i++)
#define dec(i,l,r) for(int i=l;i>=r;i--)
#define link(x) for(edge *j=h[x];j;j=j->next)
#define mem(a) memset(a,0,sizeof(a))
#define ll long long
#define eps 1e-8
#define succ(x) (1<<x)
#define lowbit(x) (x&(-x))
#define sqr(x) ((x)*(x))
#define mid (x+y)/2
#define NM 400005
#define nm 105
using namespace std;
const double pi=acos(-1);
const ll inf=998244353;
ll read(){
ll x=0,f=1;char ch=getchar();
while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
while(isdigit(ch))x=x*10+ch-'0',ch=getchar();
return f*x;
}
int n,c[NM],m,rev[NM],_x;
ll ans;
#define cp complex<double>
cp a[NM];
void fft(cp*a,int f){
inc(i,0,n-1)if(i<rev[i])swap(a[i],a[rev[i]]);
for(int k=1;k<n;k<<=1){
cp t(cos(pi/k),f*sin(pi/k));
for(int i=0;i<n;i+=k<<1){
cp w(1,0);
for(int j=0;j<k;j++,w*=t){
cp x=a[i+j],y=w*a[i+j+k];
a[i+j]=x+y;a[i+j+k]=x-y;
}
}
}
}
void plu(cp*a){
int bit=0;
n<<=1;n--;
while(succ(bit)<n)bit++;n=succ(bit);
inc(i,1,n-1)rev[i]=(rev[i>>1]>>1)|((i&1)<<(bit-1));
fft(a,1);inc(i,0,n-1)a[i]*=a[i];
fft(a,-1);inc(i,0,n-1)a[i]/=n;
}
int main(){
int _=read();while(_--){
mem(a);mem(c);
scanf("%d",&n);
int cnt=0;
m=n;while(n--)scanf("%d",&_x),c[_x]++,cnt=max(cnt,_x);
n=cnt+1;ans=0;
inc(i,1,n)a[i]=c[i];
plu(a);
n=cnt+1;
inc(i,1,n)a[i*2]-=c[i];
dec(i,n,1)c[i]+=c[i+1];
inc(i,1,n)ans+=c[i]*(ll)(a[i].real()+0.5);
ans/=2;
printf("%.7lf\n",1-ans*6.0/m/(m-1)/(m-2));
}
return 0;
}
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