The first thing you should do is check your divorce agreement. This is an issue that is typically covered in a divorce agreement. If you are not sure if it was you should ask your divorce attorney.
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Go to www.irs.gov and check out Publication 501. Also, your ex-wife can allow you to claim the exemption by issuing you a form 8338. Good luck.
Marty Davidoff, email@example.com, 732-274-1600. This answer is provided for general information only. You should seek advice from an attorney or tax professional.