Tzitz Eliezer Vol 10 Siman 25 Chap 6
Tzitz Eliezer Chap 14 Siman 80
Tzitz Eliezer Chap 14 Siman 81
Titz Eliezer Chap 13 Siman 89 to 12
Shulchan Aruch Orach Chayim 329 Saif 4
Rules that absence of breathing determines death.
Shulchan Aruch Yoreh Deah 339 Paragraph 1
Gosses. Things forbidden to do to a deathly ill person that will mostly likely die in 3 days.
Double Murder
Mishnah Torah Laws of Murder Chap 9 Halacha 9
Mishna Torah Shabbat Chap 2 Halacha 19
Maomonides rules that we only check breathing.