Sigma Convergence versus Beta Convergence: Evidence from U.S.
ANDREW T. YOUNG , MATTHEW J. HIGGINS , DANIEL LEVY
Journal of Money, Credit and Banking
Volume 40, Issue 5, pages 1083–1093, August 2008
http://onlinelibrary.wiley.com/doi/10.1111/j.1538-4616.2008.00148.x/full