Anant, according to the official website https://www.bitsadmission.com/bitsatfaq.aspx if you did not secure the minimum aggregate required for BITSAT, you should appear for Improvement examinations as a whole. You cannot appear for Improvement exams in Mathematics alone. You cant even give Improvement exams only in Mathematics/Biology, Physics or Chemistry. You have to give the improvement exams in all the subjects of your 12th. If you appear for Improvement exams only for few subjects and not the entire subjects of your 12th, then you are not eligible for BITSAT. But this is not the case with JEE Mains. If you did not secure the required aggregate in your 12th, for being eligible for NITs, IIITs and CFTIs or JEE Advanced, you can appear for the improvement exams in just one subject and that is accepted. But that is not the case with BITSAT. You have to reappear for the Improvement exams in all the subjects that you have studied in your 12th and not just one or two subjects.