title: How can I calculate inverse of sparse matrix in Eigen library
body: <p>I have a question about Eigen library in C++. Actually, I want to calculate inverse matrix of sparse matrix. 
  When I used Dense matrix in Eigen, I can use .inverse() operation to calculate inverse of dense matrix. 
  But in Sparse matrix, I cannot find inverse operation anywhere. Does anyone who know to calculate inverse of sparse matrix? help me. </p>