\(\newcommand{L}[1]{\| #1 \|}\newcommand{VL}[1]{\L{ \vec{#1} }}\newcommand{R}[1]{\operatorname{Re}\,(#1)}\newcommand{I}[1]{\operatorname{Im}\, (#1)}\)

Not a number

>>> import numpy as np
>>> np.nan
nan
>>> np.array(0) / 0
nan
>>> np.nan == 0
False
>>> np.nan == np.nan
False
>>> np.isnan([0, np.nan])
array([False,  True], dtype=bool)