J.E. Hirsch: "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."

J.E. Hirsch, An index to quantify an individual's scientific research output
PNAS 2005; 102: 16569-16572
Department of Physics, University of California at San Diego, La Jolla, CA 92093-0319

L. Egghe: "Note that it is a consequence of the definition of the h-index that the top-h papers have at least h² citations but that the actual number can be much higher (this is what is missing in the h-index). We therefore define the g-index as the highest number g of papers that together received g² or more citations. From this definition it is already clear that g &ge h."

L. Egghe, An improvement of the h-index: the g-index ; ISSI Newsletter 2:1 (2006) 8-9
L. Egghe, Theory and practise of the g-index ; Scientometrics, 69:1 (2006) pp. 131-152
University of Hasselt, Belgium

R. Rousseau: "Taken together, g and h present a concise picture of a scientist's achievements in terms of publications and citations"

R. Rousseau, New developments related to the Hirsch index ; Industrial Sciences and Technology, Belgium

S. Alonso, F.J. Cabrerizo, E. Herrera-Viedma, F. Herrera: "We present a combined index, that we call the hg-index that tries to fuse all the benefits of both previous measures and that tries to minimize the drawbacks that each one of them. The hg-index of a researcher is computed as the geometric mean of his h- and g- indices"

S. Alonso, F.J. Cabrerizo, E. Herrera-Viedma, F. Herrera, hg-index: A new index to characterize the scientific output of researchers based on the h- and g- indices ; Scientometrics, 82:2 (2010) pp. 391-400
University of Granada, Spain