Difference equations applied to DSP I've found almost nothing about difference equations on the internet. Can you please recommend me something like books or pdf online that handle deep this topic? I'm searching also some exercises. Thank you!
https://ccrma.stanford.edu/~jos/filters/ and specifically
https://ccrma.stanford.edu/~jos/fp/Difference_Equation_I.html 
http://www.dspguide.com/pdfbook.htm
https://www.amazon.com/Digital-Signal-Processing-Alan-Oppenheim/dp/0132146355
    (need to buy the book)