TITLE: ODE introduction textbook
QUESTION [6 upvotes]: Unfortunately I have reached the maximum number of math classes I can take for my undergraduate degree.  I still wish to study basic ODEs and basic number theory.  What is a good textbook with an introduction to these?  I would prefer a textbook that is not super rigorous or formal since I will be studying it on my own time.
Thank you.

REPLY [2 votes]: For learning ODE's, a popular undergraduate book is 

1.)Differential Equations with Boundary Value problems-Polking, Bogges, Arnold.

If you want to see many examples, I recommend you get the 

2.)Schaum's Outline on Differential equations-Bronson,Costa.

From personal experience I highly recommend both of these books. If you want to see slightly more advance topics with a geometric taste I recommend

3.) Ordinary Differential Equations-V.I. Arnold

As for Number theory, if you want a computational approach, consider

1.) Elementary Number Theory-Burton

For a more theoretical approach,

2.) A classical Introduction to Modern Number theory-Rosen, Ireland
  3.) Introduction to Analytic Number Theory-Apostol