Permanence of Mathematical Relations Principle
The metric properties discovered for a primitive figure remain applicable, without modifications other than changes ofsigns, to all correlative figures which can be considered to arise from the first.
This principle was formulated by Poncelet, 
and amounts to the statement that if an analytic identity in anyfinite number of variables holds for all real values of the variables, then it also holds by Analytic Continuation forall complex values (Bell 1945). This principle is also called Poncelet's Continuity Principle.
References
Bell, E. T. The Development of Mathematics, 2nd ed. New York: McGraw-Hill, p. 340, 1945.
- Equinumerous
- Equipollent
- Equipotent
- Equipotential Curve
- Equiproduct Point
- Equireciprocal Point
- Equirectangular Projection
- Equiripple
- Equitangential Curve
- Equivalence Class
- Equivalence Problem
- Equivalence Relation
- Equivalent
- Equivalent Matrix
- Eratosthenes Sieve
- Erdös-Anning Theorem
- Erdös-Kac Theorem
- Erdös-Mordell Theorem
- Erdös Number
- Erdös Reciprocal Sum Constants
- Erdös-Selfridge Function
- Erdös Squarefree Conjecture
- Erdös-Szekeres Theorem
- Erf
- Erfc