Automorphism groups of rigid complete intersections

We study the automorphism groups of complete intersections of hypersurfaces of strictly increasing degrees in projective space.

Under a combinatorial rigidity condition on the tuple of defining polynomials, we show that every automorphism of the complete intersection extends to an automorphism of each defining hypersurface, so that its automorphism group is the intersection of the automorphism groups of the defining hypersurfaces.

We apply this principle to two natural families of complete intersections of two hypersurfaces of different degrees.

For complete intersections of two Fermat hypersurfaces, we determine the automorphism group in every smooth case.

For complete intersections of a Klein hypersurface with the reverse-order Klein hypersurface, we describe the automorphism group under an explicit arithmetic condition relating the two degrees, with Klein hypersurfaces of Wagstaff type as a natural source of examples.