Anita Saha Madhumangal Pal Maximum weight k-independent set problem on permutation graphs. 1477-1487 2003 80 Int. J. Comput. Math. 12 http://taylorandfrancis.metapress.com/openurl.asp?genre=article&issn=0020-7160&volume=80&issue=12&spage=1477 db/journals/ijcm/ijcm80.html#SahaP03