First variation of area
With
A(X,Y)=(∇XY)⊥ and
H=trA,
a variation with
velocity V satisfies
dtdArea(Mt)t=0=−∫M⟨H,V⟩,dμ+∫∂M⟨V,η⟩,dσ. Thus
a boundaryless
submanifold is stationary for every compactly supported
variation exactly when it is minimal.