When Å·ÃÀ¿Ú±¬ÊÓƵ-Boulder Professor Emeritus Albert Bartlett first delivered his celebrated lecture on "Arithmetic, Population and Energy" to a group of Å·ÃÀ¿Ú±¬ÊÓƵ students on Sept. 19, 1969, the world population was close to 3.7 billion. On Oct. 12, the United Nations projects that the world population will reach 6 billion. Bartlett, an award-winning University of Å·ÃÀ¿Ú±¬ÊÓƵ at Boulder physics professor, now has marked the 30th anniversary of his lecture. He delivered the talk for the 1,325th time near Jamestown, Colo., on Sept. 19 to the Rocky Mountain chapter of the Sierra Club.