Hello

One can choose any subject of his or her choice for optional in UPSC examination

So yes you can definitely choose mathematics as your optional subject in your UPSC examination

UPSC 2018 topper ,Kanishk Kataria has mathematics as optional.

Also,

Follow the below tips to prepare well for your UPSC Examination:-

Go through syllabus well , complete it at least 3 months before Examination so that you can keep the last months for revision as the syllabus of UPSC is quite wide

Top books which you can follow for UPSC examination preparation are listed below subject wise:-

HISTORY

NCERT XI (Ancient & Medieval)
NCERT XII (Modern Indian History)
Facets of Indian Culture Spectrum
From Plassey to Partition - A History of Modern India

Indian Art & Culture

Indian Art & Culture - Nitin Singhania

Polity

NCERT IX-XII
Indian Polity M Laxmikanth

Geography

NCERT VI X (Old Syllabus)
NCERT XI, XII (New Syllabus)
Certificate Physical Geography G C Leong
World Atlas

Economics

NCERT XI
The Indian Economy Sanjiv Verma
Indian Economy by Ramesh Singh
Economic Development & Policies in India Jain & Ohri

Environment

Class XII Biology (last 5 chapters)
Environment by Shankar IAS Academy
Environment & Ecology: Biodiversity, Climate Change and Disaster Management - General Studies for Civil Services (Main) Examination - Paper II and IV - Majid Hussain

International Relations

NCERT XII (Contemporary World Politics)
Current Affairs ( read newspaper as well)

To get the complete list of books go through
https://competition.careers360.com/articles/books-for-upsc-ias

• Make a routine or a time table and try to give at least 8 hours to studies , keep short brakes of 20 to 30 minutes after every 2.5 to 3 hours
• Read newspaper daily and keep yourself updated with the current affairs
• Make short notes after completing each chapter

Revise :- there's no point of preparing for the exam if you couldn't recall anything in the UPSC Examination so you should stress on revision as well.

Solve previous year question papers and mock test papers ;- You should solve these , it will give you idea of types of questions asked and their difficulty level and will help you a lot.

Also work on your communication skills and personality development as it plays an important role in your interview round

For more tips go through

https://competition.careers360.com/articles/how-prepare-for-upsc-ias

Thankyou