MATLAB Eigen Notes 
[Y,IX] = sort(Y,dim,mode)
 
igl::sort(X,dim,mode,Y,IX)
 MATLAB version allows Y to be a multidimensional matrix, but the Eigen version is only for 1D or 2D matrices. 
B(i:(i+w),j:(j+h)) = A(x:(x+w),y:(y+h))
 
B.block(i,j,w,h) = A.block(i,j,w,h)
 MATLAB version would allow w and h to be non-positive since the colon operator evaluates to a list of indices, but the Eigen version needs non-negative width and height values. 
max(A(:))
 
A.maxCoeff()
 Find the maximum coefficient over all entries of the matrix. 
min(A(:))
 
A.minCoeff()
 Find the minimum coefficient over all entries of the matrix.