#include <bits/stdc++.h>
using namespace std;
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t; cin>>t;
    while(t--){
        int n; cin>>n;
        vector<int>a(n);
        for(int &x : a) cin>>x;
        set<int>di;
        for(int i=0;i<n;++i){
            for(int j=i+1;j<n;++j){
                int d=abs(a[i]-a[j]);
                for(int k=1; k*k<=d;++k){
                    if(d%k==0) {
                        di.insert(k);
                        di.insert(d/k);
                    }
                }
            }
        }
        int res=1;
        for(int g : di){
            unordered_map<int, int>freq;
            for(int x : a) freq[x%g]++;
            for(auto p : freq) if(p.second>1) res=max(res, g);
        }
 
        cout<<res<<endl;
    }
    return 0;
}
 

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