1. Robert G. Bartle and Donald R. Sherbert, Introduction to Real Analysis. John Wiley & Sons, Inc., Fourth Edition, 2011.
2. Stephen Abbott, Understanding Analysis. Springer, Second Edition, 2015.
3. Michael Spivak, Calculus. Publish or Perish, Inc., Third Edition, 1994.
4. https://mathworld.wolfram.com/DirichletFunction.html