Horatio, a citizen and resident of Ireland, is an equal partner with Imogene, a US citizen, in a consulting firm operating out of a Miami office and serving clients located in Florida. Under the terms of the K agreement, Horatio and Imogene are to share equally in profits and losses. During the course of the year, Horatio devotes 100 days to the venture, but works primarily at home in Dublin and communicates with his partner and his clients by telephone, fax, and Internet. In fact, Horatio is actually in the United States only 10 days during the year. During the year, the K earns net income of $200,000, all of which derives from the provision of consulting services by the K to clients in the United States. All of the K income is reinvested in the business and none is distributed to the partners. Horatio has no other economic involvement in the United States.

To what extent, if at all, will Horatio be subject to US income taxes for the year?