Intertwinors on Differential Forms over the Product of Spheres
  We give explicit formulas for the intertwinors on the differential form bundles over $S^{p-1} \times S^{q-1}$ with the standard pseudo-Riemannian metric $g=-g_{{}_{S^{p-1}}}+g_{{}_{S^{q-1}}}$ of signature $(p-1,q-1)$. As a special case, we construct conformally invariant differential operators of all even orders. 
Doojin Hong
math.DG