double **mmul(double **a,double **b,int n) {
      double **c;
      c=new double*[n];
      for (int i=0; i<n;i++) {
          c[i]=new double[n];
      }
      for (int i=0;i<n;i++) {
         for (int j=0;j<n;j++) {
             for (int k=0;k<n;k++) {
                c[i][j]+=a[i][k]*b[k][j];
             }
         }
      }
      return c;
      }