A nonlinear function is of power type with exponent if one has the bounds
(so in particular ) and
for all and some constant . Note that the first bound is a special case of the second once one assumes that . If F is continuously differentiable, then the second bound is equivalent to the bound
(possibly for a slightly different value of C), thanks to the fundamental theorem of calculus identity
The model example of a power type nonlinearity is the pure power nonlinearity (for either real or complex u). If p is an integer, any function F which is a homogeneous polynomial of degree p in u and (in the complex case) also qualifies as a power type nonlinearity. As a rule of thumb, local well-posedness results which hold for pure power nonlinearities, also hold for power type nonlinearities of the same exponent. However, in the high regularity theory, it is often necessary to impose additional hypotheses on F, for instance that is a power type nonlinearity of order p-1.
Special types of power nonlinearity
A power type nonlinearity is U(1)-invariant if
for all real and complex u. In the context of the NLS, this ensures that the equation enjoys both phase rotation invariance and Galilean invariance. Thus the pure power nonlinearities are U(1)-invariant, but other polynomials such as or are not.
A power type nonlinearity is conservative or Hamiltonian if there exists a potential such that . This definition can be complexified by identifying complex spaces with real spaces of twice the dimension. For instance, the pure power nonlinearities are Hamiltonian with potential . Normalizing , we see that the potential V is thus a power type nonlinearity of order . A Hamiltonian nonlinearity is U(1)-invariant if and only if its potential function V(u) is radial, i.e. it depends only on the magnitude |u| of u.
We say that a Hamiltonian nonlinearity is coercive if the potential is non-negative modulo lower order terms. Thus for instance, the defocusing nonlinearity is coercive, but the focusing nonlinearity is not.