include <Eigen/Dense> succeed, include <Eigen> failed - c++

Question

My question is similar to this 5 years old question.
I can successfully compile it if I use the following code:
#include <Eigen/Dense>
#include <iostream>
//#include <boost/serialization/array_wrapper.hpp>
#include <boost/date_time/gregorian/gregorian.hpp>
int main(){
std::cout<<"asfserea"<<std::endl;
boost::gregorian::date d(boost::gregorian::day_clock::local_day());
std::cout << d.year() << d.month() <<d.day() << std::endl;
return 0;
}
But I got errors after I changed annotation from #include <Eigen/Dense> to #include <Eigen>, from #include <boost/date_time...> to #include <boost/serialization...>:

Answer 1

File you are looking for is <Eigen/Eigen>. If you want to include <Eigen>, you should add correct folder to your include path.
For array_wrapper.hpp, you need newer version of Boost. At least version 1.67.0 has this file.

Answer 2

When you #include <Eigen>, your compiler looks at the directories that you have told it about, for a file named Eigen. It only finds a folder named Eigen, and within that folder there are files Eigen and Dense. It does not look inside that folder.
If you change your compilers parameters to include the Eigen folder (rather than it's parent), you can #include <Eigen> and #include <Dense>. Alternatively, you can keep your include directories as they are, and prepend Eigen/ to all the Eigen includes