Eruvin 5:9-6:1

Eiruvin 5:9

Rabbi Akiva said to the Sages, “Don’t you agree with me that a person who puts his eiruv in a cave only has 2,000 cubits from the location of the eiruv?” They replied, “That’s only when it’s not fit to live in. If it’s suitable to live in, he may walk the entirety of the cave plus 2,000 cubits beyond it.” We see from this that the interior of the cave is more lenient that the exterior. The Sages agree with Rabbi Akiva in the case of a person measuring his Shabbos limit that ends in the middle of a cave, that he only has 2,000 cubits (and not 2,000 cubits plus the entire cave).

Eiruvin 6:1

If a Jew shares a courtyard with a non-Jew, or a person who does not recognize the legal validity of an eiruv (i.e., a Samaritan), that other resident impedes his ability to make an eiruv. This is the opinion of Rabbi Meir; Rabbi Eliezer ben Yaakov says that a Jew can only be impeded from making an eiruv by another Jew.