שינויים
/* העשרה */
*[https://en.wikibooks.org/wiki/Topology Topology in Wikipedia]
*[(H. Furstenberg) https://en.wikipedia.org/wiki/Furstenberg%27s_proof_of_the_infinitude_of_primes 's_proof_of_the_infinitude_of_primes Topological proof of the infinitude of primes]
