I am trying to install eigen, but I don't seem to get it to work.

I did:

```
sudo apt-get install libeigen3-dev
```

and everything seems fine, after

```
dpkg -p libeigen3-dev
```

I get:

```
Package: libeigen3-dev
Priority: extra
Section: libdevel
Installed-Size: 3718
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Architecture: all
Source: eigen3
Version: 3.2.0-4
Depends: pkg-config
Suggests: libeigen3-doc
Size: 698062
Description: lightweight C++ template library for linear algebra
Eigen 3 is a lightweight C++ template library for vector and matrix math,
a.k.a. linear algebra.
.
Unlike most other linear algebra libraries, Eigen 3 focuses on the simple
mathematical needs of applications: games and other OpenGL apps, spreadsheets
and other office apps, etc. Eigen 3 is dedicated to providing optimal speed
with GCC. A lot of improvements since 2-nd version of Eigen.
Original-Maintainer: Debian Science Maintainers <debian-science-maintainers@lists.alioth.debian.org>
Homepage: http://eigen.tuxfamily.org
```

Everything looked fine to me. However, when I try to compile the basic code (given in the tutorial):

first_eigen.cpp

```
#include <iostream>
#include <Eigen/Dense>
using namespace Eigen;
int main()
{
Matrix2d a;
a << 1, 2,
3, 4;
MatrixXd b(2,2);
b << 2, 3,
1, 4;
std::cout << "a + b =\n" << a + b << std::endl;
std::cout << "a - b =\n" << a - b << std::endl;
std::cout << "Doing a += b;" << std::endl;
a += b;
std::cout << "Now a =\n" << a << std::endl;
Vector3d v(1,2,3);
Vector3d w(1,0,0);
std::cout << "-v + w - v =\n" << -v + w - v << std::endl;
}
```

I run it in the shell like this:

```
g++ -std=c++11 first_eigen.cpp -o my_exec
```

I get the following error:

```
first_eigen.cpp:2:23: fatal error: Eigen/Dense: No such file or directory
#include <Eigen/Dense>
^
compilation terminated.
```

So it looks like `eigen`

was not installed. What am I missing?