Best books for learning the maths behind neural networks (free?)

I’m currently trying to study deep learning outside of university, and I want to read and have a better understanding of some of the mathematical concepts behind neural networks

Is there any books people would recommend (preferably free, my budget isn’t that high) on this concept

Also I’m still relatively new to understanding deep learning, I took a few modules In university and made my way through “deep learning with python”, is there any other general books or resources people …

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