Nonlinear scalar field equations with $L^2$ constraint: Mountain pass and symmetric mountain pass approaches

We study the existence of radially symmetric solutions of the following nonlinear scalar field equations in ${\mathbb R}^N$ ($N\geq 2$): (*)_m \left\{ \eqalign{ -&\Delta u = g(u) -\mu u \quad \hbox{in}\ {\mathbb R}^N, \cr &\| u\|_{L^2({\mathbb R}^N)} = m, \cr &u \in H^1({\mathbb R}^N), \cr} \right. where $g(\xi)\in C({\mathbb R},{\mathbb R})$, $m>0$ is a given constant and $\mu\in {\mathbb R}$ is a Lagrange multiplier. We introduce a new approach using a Lagrange formulation of the problem $(*)_m$. We develop a new deformation argument under a new version of the Palais-Smale condition. For a general class of nonlinearities related to [BL1, BL2, HIT], it enables us to apply minimax argument for $L^2$ constraint problems and we show the existence of infinitely many solutions as well as mountain pass characterization of a minimizing solution of the problem: $$\inf\left\{ \int_{{\mathbb R}^N} {1\over 2}|\nabla u|^2 - G(u)\, dx;\, \| u\|_{L^2({\mathbb R}^N)}^2 = m \right\}, \quad G(\xi)=\int_0^\xi g(\tau)\, d\tau.$$