Goldblatt, Robert (1998). "Exercise 5.7 (4)". Lectures on the Hyperreals: An introduction to nonstandard analysis. Graduate Texts in Mathematics. Vol. 188. New York: Springer-Verlag. p. 54. doi:10.1007/978-1-4612-0615-6. ISBN0-387-98464-X. MR1643950.
Goldblatt, Robert (1998). "Exercise 5.7 (4)". Lectures on the Hyperreals: An introduction to nonstandard analysis. Graduate Texts in Mathematics. Vol. 188. New York: Springer-Verlag. p. 54. doi:10.1007/978-1-4612-0615-6. ISBN0-387-98464-X. MR1643950.