Array< double, 1, 3 > e(2,-3, 1./3.)
const Eigen::CwiseBinaryOp< Eigen::internal::scalar_pow_op< typename Derived::Scalar, typename ExponentDerived::Scalar >, const Derived, const ExponentDerived > pow(const Eigen::ArrayBase< Derived > &x, const Eigen::ArrayBase< ExponentDerived > &exponents)