In this paper we examine the linear elliptic partial differential operators that appear as Euler-Lagrange systems of certain variational integrals. We give a sufficient condition for those systems to be deformable to the Laplace system.
"Homotopic Classification of Euler-Lagrange Systems." Missouri J. Math. Sci. 14 (1) 49 - 56, Winter 2002. https://doi.org/10.35834/2002/1401049