int N = a.length;
double [][] c = new double[N][N];

for (int i = 0; i<N ;i++){
for(int j = 0; j<N; j++){
//计算行i和列j的点乘
for(int k=0; k<N;k++){
c[i][j] += a[i][k]*b[k][j];
}
}
}