In Sukkah, Daf 17, in the chart and the insights, you write that according to RABAH (SURA) and RAV (NEHERDA'A), there is no such Halachah l'Moshe mi'Sinai of "Dofen Akumah." Rather, they hold that there is a Halachah l'Moshe mi'Sinai stating that this is the amount of invalid Sechach that will invalidate a Sukah (RASHI 17b, DH Mishum).

It seemed to us in shiur that there is not sufficient justification to say this. I don't read the Rashi that way, and you would have to find a good reason why the mishnah speaks about 3 tephahim and 4 amos from the side.

We came up with some possible differences between 4 amos in the middle and 4 at the side of the sukkah. For example, according to the Ohr Somayach, even though you can have up to 4 amos of posul schach in the sukkah, the sukkah must still have a majority of schach kosher. However, because of dofen akumah, posul schach at the side need not be included in this calculation. It's wall, not schach.

Indeed, one could argue that the mishnah gave the case of 4 amos from the side precisely in order not to complicate the issue with the question of whether there remained a majority schach kasher.

Be well,

Shlomo Zalman