ldlt

代码说明：

  在实际计算中，有时为了避免开方运算，或者要分解的矩阵只是对称而不是正定的，这时可以选用适合于一般对称阵的分解式(In the actual calculation, sometimes in order to avoid root operation, or just want to break down the matrix rather than positive definite symmetric, then you can choose suitable for general symmetric matrix factorization)