Derrick's theorem beyond a potential
  Scalar field theories with derivative interactions are known to possess
solitonic excitations, but such solitons are generally unsatisfactory because
the effective theory fails precisely where nonlinearities responsible for the
solitons are important. A new class of theories possessing (internal) galilean
invariance can in principle bypass this difficulty. Here, we show that these
galileon theories do not possess stable solitonic solutions. As a by-product,
we show that no stable solitons exist for a different class of derivatively
coupled theories, describing for instance the infrared dynamics of superfluids,
fluids, solids and some k-essence models.
