Fundamental Lemma of Calculus of Variations
If
with Continuous second Partial Derivatives, then 
on the Open Interval
.- Tetrad
- Tetradecagon
- Tetradecahedron
- Tetradic
- Tetradyakis Hexahedron
- Tetraflexagon
- Tetragon
- Tetrahedral Coordinates
- Tetrahedral Graph
- Tetrahedral Group
- Tetrahedral Number
- Tetrahedral Surface
- Tetrahedroid
- Tetrahedron
- Tetrahedron 10-Compound
- Tetrahedron 5-Compound
- Tetrahedron Inscribing
- Tetrahemihexacron
- Tetrahemihexahedron
- Tetrakaidecagon
- Tetrakaidecahedron
- Tetrakis Hexahedron
- Tetranacci Number
- Tetrix
- Tetromino