J. E. Hirsch* . J. E. Hirsch* Department of Physics, University of California at San Diego, La Jolla, CA 92093-0319 PNAS November 15, 2005  vol. 102  no. 46  16569–16572. I propose the index h, defined as the number of papers with citation number >h, as a useful index to characterize the scientific output of a researcher