[citizensoldier]

I have been researching Title 26 of the USC and have found the following:

Subtitle F - Procedure and Administration
Chapter 29 - Definitions

(a) When used in this title, where not otherwise distinctly
expressed or manifestly incompatible with the intent thereof -

(1) Person
The term ''person'' shall be construed to mean and include an
individual, a trust, estate, partnership, association, company or
corporation.
(9) United States
The term ''United States'' when used in a geographical sense
includes only the States and the District of Columbia.
10) State
The term ''State'' shall be construed to include the District
of Columbia, where such construction is necessary to carry out
provisions of this title.
(26) Trade or business
The term ''trade or business'' includes the performance of the
functions of a public office.
(30) United States person
The term ''United States person'' means -
(A) a citizen or resident of the United States,
(B) a domestic partnership,
(C) a domestic corporation,
(D) any estate (other than a foreign estate, within the
meaning of paragraph (31)), and
(E) any trust if -
(i) a court within the United States is able to exercise
primary supervision over the administration of the trust, and
(ii) one or more United States persons have the authority
to control all substantial decisions of the trust.

b) Definition of resident alien and nonresident alien
(1) In general
For purposes of this title (other than subtitle B) -
(A) Resident alien
An alien individual shall be treated as a resident of the
United States with respect to any calendar year if (and only
if) such individual meets the requirements of clause (i), (ii),
or (iii):
(i) Lawfully admitted for permanent residence
Such individual is a lawful permanent resident of the
United States at any time during such calendar year.
(ii) Substantial presence test
Such individual meets the substantial presence test of
paragraph (3).
(iii) First year election
Such individual makes the election provided in paragraph
(4).
(B) Nonresident alien
An individual is a nonresident alien if such individual is
neither a citizen of the United States nor a resident of the
United States (within the meaning of subparagraph (A)).

I do not see any definition of the United States in Subtitle A Chapter 1. Where can I find the definition defining the United States as anything other than its geographic reference as defined above in determining what the IRS deems is a citizen or resident of the United States?