Pressley, Andrew (2010). 《Elementary Differential Geometry》. Springer Undergraduate Mathematics Series 2판. London: Springer-Verlag. Definition 2.1.1, p. 30 and Exercise 2.2.6, p. 44. doi:10.1007/978-1-84882-891-9. ISBN978-1-84882-890-2. MR2598317.
Pressley, Andrew (2010). 《Elementary Differential Geometry》. Springer Undergraduate Mathematics Series 2판. London: Springer-Verlag. Definition 2.1.1, p. 30 and Exercise 2.2.6, p. 44. doi:10.1007/978-1-84882-891-9. ISBN978-1-84882-890-2. MR2598317.