There are several journals and publications that provide solutions to complex problems in the UPSC Mathematics Optional subject. These resources can be extremely helpful for students preparing for the UPSC examination. Some of the notable journals and publications in this field are:

1. "Mathematics Today": This monthly journal covers a wide range of topics in mathematics and also includes solutions to complex problems. It provides a comprehensive understanding of various mathematical concepts and is particularly useful for UPSC Mathematics Optional subject preparation.

2. "Ramanujan Journal": This esteemed journal publishes high-quality research papers in mathematics. Although it may not specifically provide solutions to problems related to the UPSC examination, it offers advanced mathematical content that can enhance the problem-solving skills of aspirants.

3. "UPSC Mathematics Optional Solved Papers": This publication is specifically designed for UPSC Mathematics Optional subject aspirants. It contains solved papers from previous years' examinations, allowing students to practice and understand the pattern of questions asked in the UPSC examination.

4. "UPSC Mathematics Optional: Topic-wise Solved Papers": This book provides topic-wise solutions to complex problems in the UPSC Mathematics Optional subject. It covers a wide range of topics such as linear algebra, calculus, differential equations, and more. This resource is highly recommended for systematic preparation and practice.

5. "Mathematics for IAS Main Examination": This comprehensive book covers all the topics required for the UPSC Mathematics Optional subject. It includes detailed explanations and solutions to complex problems, making it an essential resource for aspirants.

6. "UPSC Mathematics Optional: Previous Years' Solved Papers": This publication contains solved papers from previous years' UPSC Mathematics Optional examinations. It provides aspirants with an opportunity to analyze the types of questions asked and practice solving them.

7. "Mathematics Optional: Topic-wise Solved Papers": This book focuses on providing topic-wise solutions to problems in the UPSC Mathematics Optional subject. It covers various topics in detail, ensuring that aspirants have a strong foundation in each area.

In conclusion, these journals and publications offer valuable resources for UPSC Mathematics Optional subject aspirants. They provide solutions to complex problems, previous years' question papers, and topic-wise practice material, enabling students to thoroughly prepare for the examination.