2 rules
an individual who is married (within the meaning of subsection (a)) and who files a separate return maintains as his home a household which constitutes for more than one-half of the taxable year the principal place of abode of a child (within the meaning of section 152(f)(1)) with respect to whom such individual is entitled to a deduction for the taxable year under section 151 (or would be so entitled but for section 152(e)),