I'm a first year econometrics student with a great interest in mathematics. I very much enjoy my study, but still I am interested to learn about more topics in mathematics which are not part of my study. Some of the main topics which I am already familar with or which will soon be covered in my study are: calculus, linear algebra, optimization, statistics/probability/combinatorics.
Which topics in mathematics would you advise me to study still (that are not part of this list)? I have a general interest in mathematics, so any advice on interesting and or essential topics in mathematics that are worth studying is appreciated.
If possible, could you also give me advice on books/references which I should study from?
Thanks in advance.
Edit: To be more specific, I am looking for topics on which the general consensus is that they are essential to know of for any mathematician or very interesting to study.
You'll just need to get some universities syllabi an pick some topics to study. The syllabi do have a lot of subjects but they also provide you an order in which they should be studied that will restrain a little of your choices. You won't be lost with a diversity of fields of study because you'll have to cover some fundamentals first.
Cambrige and Oxford have nice materials for guiding your study - It'll also be useful in the case that you know some of the first subjects, you'll be able to pick more advanced stuff. The folowing resources are going to be very useful:
How to Become a Pure Mathematician
This website has book recomendations and also information on what order it should be studied.
All the Mathematics You Missed: But Need to Know for Graduate School
This book comments on the importance of some math fields for the mathematical study.
Cambridge Syllabus
Oxford Syllabus
I've found both syllabi enlightening and informative, with them you're going to have something similar to the site I mentioned before: fields of study, the order in which they should be studied, some intruction on what you should be able to do after covering the topics and a little about the importance of them.
There are also some all-in-one books and book collections that you should look:
Mathematics, It's Content Method and Meaning
Fundamentals of Mathematics
The World of Mathematics
Mathematics Form and Function
What it Mathematics?
Princeton Companion to Mathematics
For the end, as a personal suggestion: Don't get afraid, just get the books and start reading, when the things start to become dark you can use the torches of our fellow members to lighten your path! Good Luck!