Three-circle inequalities for almost harmonic functions on cylinders
The three-circle method was introduced into the blow-up analysis of harmonic maps by Qing and Tian [Comm. Pure Appl. Math. 1997]. We extend this method to a class of $(τ,ε)$-almost harmonic functions on long cylinders and establish three-circle inequalities for their weighted energies. These inequalities yield a dichotomy between exponential energy decay and asymptotic dominance of a critical Fourier mode, together with a characterization of the corresponding normalized limiting profiles. In the unweighted case, we prove convergence of the averaged neck curves and identify their limiting equations under additional structural assumptions. Applications include neck analysis and energy identities for harmonic maps, solutions of Rivière's equation, $α$-harmonic maps, biharmonic approximations of harmonic maps, and Willmore surfaces.