Some of the reputed institutes offering this courses are IITs, IT BHU, Indian Institute of Science Education and Research (IISER) Pune, Delhi University etc. all of these institutes require entrance test along with post graduation degree for PhD. However you may explore IIT Guwahati (http://www.iitg.ernet.in/maths/acads/phd.php) offering PhD in Maths. Minimum Qualification is M.Sc. and Selection Criteria is Interview conducted twice a year (Dec & June). You may also explore Indira Gandhi National Open University, (New Delhi) for PhD in Mathematics. Depending upon your specialization and based on availability and other eligibility criteria you can get enrolled in this program. In general minimum qualification for PhD is post graduation degree.