A metric characterizing the difference between two knots 
and 
in 
.
Gordian Distance
References
Murakami, H. "Some Metrics on Classical Knots." Math. Ann. 270, 35-45, 1985.Referenced on Wolfram|Alpha
Gordian DistanceCite this as:
Weisstein, Eric W. "Gordian Distance." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/GordianDistance.html