This work is in the public domain in the United States because it meets three requirements:

1. it was first published outside the United States (and not published in the U.S. within 30 days),
2. it was first published before 1 March 1989 without copyright notice or before 1964 without copyright renewal or before the source country established copyright relations with the United States,
3. it was in the public domain in its home country （Vietnam） on the URAA date (23 December 1998).