In this work we consider equations of the form
84.0pt∂tubadbreak+Ptrue(∂xtrue)ugoodbreak+Gtrue(u,∂xu,⋯,∂xlutrue)=0,$$\begin{equation*}\hskip7pc \partial _t u+P\big (\partial _x\big ) u+G\big (u,\partial _xu,\dots ,\partial _x^l u\big )=0, \end{equation*}$$where P is any polynomial without constant term, and G is any polynomial without constant or linear terms. We prove that if u is a sufficiently smooth solution of the equation, such that prefixsuppufalse(0false),prefixsuppufalse(Tfalse)⊂(−∞,B]$\operatorname{supp}u(0),\operatorname{supp}u(T)\subset { (-\infty ,B ]}$ for some B>0$B>0$, then there exists R0>0$R_0>0$ such that prefixsuppu(t)⊂(−∞,R0]$\operatorname{supp}u(t)\subset (-\infty ,R_0]$ for every t∈false[0,Tfalse]$t\in [0,T]$. Then, as an example of the application of this result, we employ it to show a unique continuation principle for the Kawahara equation,
108.0pt∂tubadbreak+∂x5ugoodbreak+∂x3ugoodbreak+u∂xugoodbreak=0,$$\begin{equation*}\hskip9pc \partial _t u+\partial _x^5 u+\partial _x^3 u+u\partial _x u=0, \end{equation*}$$and for the generalized KdV hierarchy
72.0pt∂tubadbreak+false(−1false)k+1∂x2k+1ugoodbreak+Gtrue(u,∂xu,⋯,∂x2kutrue)=0.$$\begin{equation*}\hskip6pc \partial _t u+ (-1)^{k+1}\partial _x^{2k+1} u+G\big (u,\partial _x u,\dots , \partial _x^{2k}u\big ) =0. \end{equation*}$$