26 U.S.C. § 7703

1. (a)For purposes of part V of subchapter B of chapter 1 and those provisions of this title which refer to this subsection—
   1. (1)the determination of whether an individual is married shall be made as of the close of his taxable year; except that if his spouse dies during his taxable year such determination shall be made as of the time of such death; and
   2. (2)an individual legally separated from his spouse under a decree of divorce or of separate maintenance shall not be considered as married.
2. (b)For purposes of those provisions of this title which refer to this subsection, if—such individual shall not be considered as married.
   1. (1)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)),
   2. (2)such individual furnishes over one-half of the cost of maintaining such household during the taxable year, and
   3. (3)during the last 6 months of the taxable year, such individual’s spouse is not a member of such household,