The math.h library for Arduino does not include a log2 function. The log2 library is an Arduino library for using logarithmic calculations with a base of 2.
The usage is straightforward: log2(value) returns a double float of log2 of the input value.
Download the library and examples here: https://github.com/little-scale/log2
To install this library, download the .zip file and move the decompressed folder to Documents/Arduino/libraries/.
The example sketch, shown below, returns 1000 Hz as a MIDI note value in the serial monitor.