Negative Mitzvah 356
It is forbidden to remarry one's divorced wife who married and divorced a second time
Deuteronomy 24:4 "Then her former (first) husband who sent her away may not take her again to be his wife"
A man who divorces his wife is forbidden to remarry her if she married another man and then, was divorced a second time or was widowed from her second husband.
(If she did not marry another man, he is permitted to marry her again.)
Positive Mitzvah 216
"Yibum"
Deuteronomy 25:5 "Her husband's brother shall marry her"
If a man dies childless, one of his brothers is obligated to marry his wife (see Negative Mitzvah 357).