A mathematical case for remaining in the EU
The mathematical case for remaining in the European Union does not rely on the arithmetic of economics. For an interesting twist, take a look at Sir Timothy Gowers' blog post at https://gowers.wordpress.com/2016/06/02/6172/. Gowers is a highly respected mathematician, and winner of the Fields Medal, and you can read more about him here: https://en.wikipedia.org/wiki/Timothy_Gowers.
In his blog post, Gowers argues that we need supranational organisations to handle scenarios in which individual countries need to co-operate in order to protect both their longer-term interest and the common good, even if this means acting against their own short-term interest or inclination. In the game theory branch of mathematics, this pattern is known as the "prisoner's dilemma". And in such situations, the long-term outcome for all parties is better if all participants co-operate rather than acting selfishly.
Using lots of real-life examples and accessible, jargon-free discussion, Gowers concludes that on an international stage, an overarching organisation (in this case, a supranational body in the form of the EU) is required in order to ensure that co-operation takes place, so that all of the member countries can benefit from a better longer-term outcome. Gowers also argues that subsidiarity is more important than sovereignty, and that decision-making should not be made purely on the basis of national interest but should also take into account the wider European interest.