Pervin (1964), p. 43, Exercise 6. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Arkhangel'skij & Fedorchuk (1990), p. 25. Arkhangel'skij, A.V.; Fedorchuk, V.V. (1990) [1988], Gamkrelidze, R.V.; Arkhangel'skij, A.V.; Pontryagin, L.S. (eds.), General Topology I, Encyclopaedia of Mathematical Sciences, vol. 17, translated by O'Shea, D.B., Berlin: Springer-Verlag, ISBN978-3-642-64767-3, LCCN89-26209.
Pervin (1964), p. 42, Exercise 5. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 46, Exercise 4. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Arkhangel'skij & Fedorchuk (1990), p. 26. Arkhangel'skij, A.V.; Fedorchuk, V.V. (1990) [1988], Gamkrelidze, R.V.; Arkhangel'skij, A.V.; Pontryagin, L.S. (eds.), General Topology I, Encyclopaedia of Mathematical Sciences, vol. 17, translated by O'Shea, D.B., Berlin: Springer-Verlag, ISBN978-3-642-64767-3, LCCN89-26209.
Pervin (1964), p. 43, Exercise 10. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 49, Theorem 3.4.3. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 60, Theorem 4.3.1. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 66, Exercise 3. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 67, Exercise 5. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 69, Theorem 5.1.1. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.
Pervin (1964), p. 70, Theorem 5.1.2. Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN9781483225159, LCCN64-17796.