http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6D-45GNJ61-H&_user=1002882&_coverDate=05%2F31%2F1980&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1336961634&_rerunOrigin=scholar.google&_acct=C000050162&_version=1&_urlVersion=0&_userid=1002882&md5=bb7573a4a79e2d48ff2a7084a69ed8a6

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V64-45F63M6-F3&_user=1002882&_coverDate=08%2F31%2F1984&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1336957694&_rerunOrigin=scholar.google&_acct=C000050162&_version=1&_urlVersion=0&_userid=1002882&md5=3ada1e8233bff52622adcbdb1d43e24c

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V6D-45JK5HW-1G&_user=1002882&_coverDate=11%2F30%2F1981&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1336958434&_rerunOrigin=scholar.google&_acct=C000050162&_version=1&_urlVersion=0&_userid=1002882&md5=34e05fd9b351b5dccba3db4b6418e5d7