IRS Publication 501 Dependents, Standard Deduction, and Filing Information page 10 - https://www.irs.gov/pub/irs-pdf/p501.pdf#page=10
Eligibility rules. You are eligible to file your 2021 return as a qualifying widow(er) if you meet all the following tests.
1. You were entitled to file a joint return with your spouse for the year your spouse died. It doesn't matter whether you actually filed a joint return.
2. Your spouse died in 2019 or 2020 and you didn't remarry before the end of 2021.
3. You have a child or stepchild (not a foster child) whom you can claim as a dependent or could claim as a dependent except that, for 2021:
a. The child had gross income of $4,300 or more,
b. The child filed a joint return, or
c. You could be claimed as a dependent on someone else’s return
4. This child lived in your home all year, except for temporary absences. See Temporary absences, earlier, under Head of Household. There are also exceptions, described later, for a child who was born or
died during the year and for a kidnapped child.
5. You paid more than half the cost of keeping up a home for the year. See Keeping Up a Home, earlier, under Head of Household.
