don't enclose eigen in a namespace. prefixing is the way to go.

For example Qt uses a prefix and no namespace (except for a few things)

src/internal/MatrixOps.h

@@ -26,8 +26,6 @@
 #ifndef EIGEN_MATRIXOPS_H
 #define EIGEN_MATRIXOPS_H
 
-namespace Eigen {
-
 template<typename Lhs, typename Rhs> class MatrixSum
   : public EigenBase<typename Lhs::Scalar, MatrixSum<Lhs, Rhs> >
 {
@@ -197,6 +195,4 @@ EigenBase<Scalar, Derived>::operator*=(const EigenBase<Scalar, OtherDerived> &ot
   return *static_cast<Derived*>(this);
 }
 
-} // namespace Eigen
-
 #endif // EIGEN_MATRIXOPS_H