Suugaku Girl

From author's site:
I love math. Just after the high school entrance ceremony I meet a beautiful girl, Milka. Milka is a mathematical genius. She gives me many math problems and shows me many elegant solutions. Milka and I spend a long time discussing math in the school library.
A year later I meet another mathematical girl, Tetra. Tetra is one year younger than me, and asks me to teach her math. While I teach her, she begins to understand math and to love its elegance gradually."

The first series is about Fibonacci Numbers, Harmonic Numbers, Finite and Infinite Sums, Factorization, The Basel Problem, Partitions of the Integer, and Generating Functions.