26 C.F.R. § 521.109
Under Article IX of the convention, a nonresident alien (including a nonresident alien individual, fiduciary, and partnership) who is a resident of Denmark, or a Danish corporation, who derives from sources within the United States in any taxable year beginning on or after January 1, 1948, income from real property (not including interest derived from mortgages or bonds secured by real property) or royalties from the operation of mines, quarries, oil wells or other natural resources may, for such taxable year, elect to be subject to Federal income tax as if such alien or corporation were engaged in trade or business within the United States by reason of having a permanent establishment therein during such taxable year. Such election shall be made by so signifying on the return for such year. The election so signified shall be irrevocable for the taxable year for which such election is made. In such case a return may be filed by the nonresident alien or foreign corporation even though the sole income of such alien or corporation from sources within the United States is fixed or determinable annual or periodical income upon which the tax has been fully satisfied at the source and there exists no necessity for the filing of the return except for the purposes of securing the benefits of Article IX of the convention. See § 29.217-2 of Regulations 111 (26 CFR 1949 ed. Supps. 29.217-2 ) [and § 39.217-2 of Regulations 118 (26 CFR, Rev. 1953, Parts 1-79, and Supps.)]
26 C.F.R. §521.109