First post. Hope this is the right place.

Using Arduino Mega 2560 had problems getting this Library to work.

Using the PCF8575 Module ( which has no output pins 8 or 9),the outputs were

OK on Pins 0 to 7 but the pins 10 to 17 were out by two places.

eg. sketch code p8 activated Pin 10. Sketch code p9 activated Pin 11 etc etc.

With the help of the Arduinp forum the problem appears to be in the .h file.

Changed the #define lines in the .h file to;

#define P00 0
#define P01 1
#define P02 2
#define P03 3
#define P04 4
#define P05 5
#define P06 6
#define P07 7
#define P10 8
#define P11 9
#define P12 10
#define P13 11
#define P14 12
#define P15 13
#define P16 14
#define P17 15
#include <math.h>

Changing the code to the above fixed the problem.

Thanks to Arduino Forum's PaulRB for the fix (https://forum.arduino.cc/index.php?topic=732201)