# Zig Zag definition

I have issues regarding the clarity of following definition from this source: http://www.cs.huji.ac.il/~nati/PAPERS/expander_survey.pdf (page 73) not sure about how g(v) is defined, and how, using this function, I can deduce the fact that $$||g||^2=||f||^2$$

Thanks.

• Can anyone here help me please? – av9w Jan 15 at 8:42