An Almgren-type formula for planar $p$-harmonic functions

For a nonconstant planar $p$-harmonic function $u$, with $1<p<\mathrm{\infty }$, and according to previous results, most notably Aronsson’s fundamental analysis of the hodograph representation in a neighborhood of an isolated critical point, we introduce the flux-normalized frequency $$N_{\ast }(x_0,r)=\frac{r{\displaystyle {\int }_{B_r(x_0)}\mathrm{\mid }Du{\mathrm{\mid }}^{p}dx}}{{\displaystyle {\int }_{\mathrm{\partial }{B}_r(x_0)}\mathrm{\mid }Du{\mathrm{\mid }}^{p-2}(u-u(x_0){)}^{2}dS}}\mathrm{.}$$ This ratio constitutes the natural counterpart of Almgren’s frequency function in the nonlinear setting, as it recovers precisely the degree of homogeneity for every homogeneous $p$-harmonic profile. We further establish the corresponding gauge-corrected version of Almgren-type monotonicity and reinterpret, in terms of the frequency function, the classical planar unique continuation property. In addition, we derive a quantitative pinching estimate for the flux-normalized frequency and identify the specific obstruction that prevents a straightforward extension of the method to higher dimensions.