1. Division of Supervision and Regulation, Federal Reserve Board of Governors, Washington, DC 20551, USA
2. Department of Finance and Accounting, Faculty of Economics and Social Sciences, University of Plovdiv Paisii Hilendarski, 4000 Plovdiv, Bulgaria
3. Department of Mathematical Analysis, Faculty of Mathematics and Informatics, University of Plovdiv Paisii Hilendarski, 4000 Plovdiv, Bulgaria