/usr/include/eigen3/Eigen/src/Core/arch/ is in libeigen3-dev 3.2.2-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/eigen3/Eigen/src/Core/arch/AltiVec/
/usr/include/eigen3/Eigen/src/Core/arch/AltiVec/Complex.h
/usr/include/eigen3/Eigen/src/Core/arch/AltiVec/PacketMath.h
/usr/include/eigen3/Eigen/src/Core/arch/Default/
/usr/include/eigen3/Eigen/src/Core/arch/Default/Settings.h
/usr/include/eigen3/Eigen/src/Core/arch/NEON/
/usr/include/eigen3/Eigen/src/Core/arch/NEON/Complex.h
/usr/include/eigen3/Eigen/src/Core/arch/NEON/PacketMath.h
/usr/include/eigen3/Eigen/src/Core/arch/SSE/
/usr/include/eigen3/Eigen/src/Core/arch/SSE/Complex.h
/usr/include/eigen3/Eigen/src/Core/arch/SSE/MathFunctions.h
/usr/include/eigen3/Eigen/src/Core/arch/SSE/PacketMath.h