The vectors 
, ..., 
in a three-space form a normalized eutactic
star iff 
for all 
in the three-space.
Hadwiger's Principal Theorem
Explore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Hadwiger's Principal Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HadwigersPrincipalTheorem.html