Let M be a projectiv variety with a rational (p,p) cycle c, then :
c=∑iqi[Mi]
with Mi, (p,p) sub-varieties and qi, rational numbers. We try to show an homotopy with algebraic sub-varieties Mi≅Ai. At this aim, we construct a flow over sub-varieties X:
∂X∂t=−grad(F)X
with F the following functional:
F(X)=∫X||J∗||2
where J∗ is the non-diagonal part of the complex structure J when we decompose the tangent space following the normal and tangent bundle of X.
We have :
F(X)=0 iff J∗=0 iff X is complex iff X is algebraic (following the GAGA theorem).
All the difficulty is to show that the flow is well defined and converges to an algebraic sub-variety, the flow will have certainly singularities which will have to be studied.