I've read it in high school and was truly fascinated with which elegance and simplicity the book approached some of the most complex math problems. Of course it just scratched the surface but partially because of this book I chose the Math Department. And our school teacher recommended Courant-Robbins to all the students in a class, many of which actually joined Math Dept too. So, it might be not just me. :)
Doesnt aldi sell unlimited sims for like 13 euro? I've lived in Hamburg now for 7 months but still use Google Fi since its also very cheap and has yet to not work in any country I go to. But I always thought you could get something SUPER cheap in Aldi.
They have a starter set for 13€ that includes sim card and 10€ credit.
They also have an XL flat rate which includes 5,5GB LTE that costs 15€/month. So you pay like 18€ in the first month and then 15€ in the following months.
This isn't bad, but I don't get O² net where I live, so I can only use Vodafone based offerings.