Optimal regularity for variational solutions of free transmission problems

In this article we study functionals of the type considered in [36], i.e. J(v):=∫B1(A(x,u)|∇u|2+f(x,u)u+Q(x)λ(u))dx here A(x,u)=A+(x)χ{u>0}+A−(x)χ{u<0}, f(x,u)=f+(x)χ{u>0}+f−(x)χ{u<0} and λ(x,u)=λ+(x)χ{u>0}+λ−(x)χ{u≤0}. We prove the optimal C0,1javax.xml.bind.JAXBElement@7bf0e485 regularity of minimizers of the functional indicated above (with precise estimates) when the coefficients A± are continuous functions and μ≤A±≤[Formula presented] for some 0<μ<1, with f∈LN(B1) and Q bounded. We do this by presenting a new compactness argument and approximation theory similar to the one developed by L. Caffarelli in [9] to treat the regularity theory for solutions to fully nonlinear PDEs. Moreover, we introduce the Ta,b operator that allows one to transfer minimizers from the transmission problems to the Alt-Caffarelli-Friedman type functionals, in small scales, allowing this way the study of the regularity theory of minimizers of Bernoulli type free transmission problems.