Hello Rajbir, 

For Ph.D. in Mathematics you can select the top colleges offering Ph.D. in Mathematics in India. The colleges are: 

• Indian Institute of Technology, New Delhi
• Indian Institute of Technology Madras, Chennai
• Indian institute of Science, Bangalore 
• Loyola College, Chennai
• Christ university, Bangalore 
• Indus university, Ahemdabad
• IIT, Gandhi Nagar 
• IFTM University, Moradabad
• Invertis University, Bareilly

Hope This Helps !