#include<cstdio>

#include<cctype>

#include<queue>

#include<cmath>

#include<cstring>

#include<algorithm>

#define rep(i,s,t) for(int i=s;i<=t;i++)

#define dwn(i,s,t) for(int i=s;i>=t;i--)

#define ren for(int i=first[x];i!=-1;i=next[i])

using namespace std;

inline int read() {

    int x=,f=;char c=getchar();

    for(;!isdigit(c);c=getchar()) if(c=='-') f=-;

    for(;isdigit(c);c=getchar()) x=x*+c-'';

    return x*f;

}

const int maxn=;

const double PI=acos(-1.0);

typedef long long ll;

struct FFT {

    struct cox {

        double r,i;

        cox(double _r=0.0,double _i=0.0) {r=_r;i=_i;}

        cox operator + (const cox& b) const {return cox(r+b.r,i+b.i);}

        cox operator - (const cox& b) const {return cox(r-b.r,i-b.i);}

        cox operator * (const cox& b) const {return cox(r*b.r-i*b.i,r*b.i+i*b.r);}

    }f[maxn];

    int len;

    void init(int* A,int L,int Len) {

        len=L;rep(i,,Len-) f[i]=cox(A[Len-i-],);rep(i,Len,L-) f[i]=cox(,);

    }

    void cal(int tp) {

        int j=len>>;

        rep(i,,len-) {

            if(i<j) swap(f[i],f[j]);int k=len>>;

            while(j>=k) j-=k,k>>=;j+=k;

        }

        double lm=-*tp*PI;

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

            cox wn(cos(lm/i),sin(lm/i));

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

                cox w(,);

                for(int k=j;k<j+(i>>);k++) {

                    cox u=f[k],v=w*f[k+(i>>)];

                    f[k]=u+v;f[k+(i>>)]=u-v;w=w*wn;

                }

            }

        }

        if(tp<) rep(i,,len-) f[i].r/=len;

    }

};

void mul(int* A,int* B,int L1,int L2,int& L,ll* ans) {

    L=;while(L<L1<<||L<L2<<) L<<=;

    static FFT a,b;a.init(A,L,L1);b.init(B,L,L2);

    a.cal();b.cal();rep(i,,L-) a.f[i]=a.f[i]*b.f[i];

    a.cal(-);rep(i,,L-) ans[i]=ll(a.f[i].r+0.5);

}

int A[maxn];

ll ans[maxn],sum[maxn];

int main() {

    int T=read();

    while(T--) {

        memset(A,,sizeof(A));

        int n=read(),v,L1=,L;

        rep(i,,n) A[v=read()]++,L1=max(L1,v+);A[]=;

        mul(A,A,L1,L1,L,ans);

        while(L>L1+L1-&&!ans[L-]) L--;

        dwn(i,L-,) sum[L--i]=ans[i];

        rep(i,,L-) sum[i]=(sum[i]-(i&?:A[i>>]))/;

        ll res=,tmp=,all=;

        dwn(i,L-,) tmp+=A[i],res+=tmp*sum[i];

        rep(i,,n) all+=(ll)(n-i)*(n-i-)/;

        printf("%.7lf\n",1.0-(double)res/all);

    }

    return ;

}