A rational homomorphism 
defined over a field is called an isogeny when 
. Two groups 
and 
are then called isogenous if there exists a third group 
and isogenies 
and 
.
Isogeny
See also
HomeomorphicExplore with Wolfram|Alpha
References
Iyanaga, S. and Kawada, Y.(Eds.). Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press, p. 47, 1980.Referenced on Wolfram|Alpha
IsogenyCite this as:
Weisstein, Eric W. "Isogeny." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Isogeny.html