In the paper the conditions for the existence of a $2\pi$-periodic solution in $t$ of the system $u_{tt}-u_{rr}-(2/r)u_r=ϵf(t,r,u,u_t,u_r)$, $\left|u(t,0)\right|<+\infty ,u(t,\pi )=0$ are investigated provided that $f$ is sufficiently smooth and $2\pi$-periodic in $t$.

In this paper, the existence of an $\omega$-periodic weak solution of a parabolic equation (1.1) with the boundary conditions (1.2) and (1.3) is proved. The real functions $f(t,r),h\left(t\right),a\left(t\right)$ are assumed to be $\omega$-periodic in $t,f\in L_2(S,H),a,h$ such that $a^{\text{'}}\in L_{\infty }\left(R\right),h^{\text{'}}\in L_{\infty }\left(R\right)$ and they fulfil (3). The solution $u$ belongs to the space $L_2(S,V)\cap L_{\infty }(S,H)$, has the derivative $u^{\text{'}}\in L_2(S,H)$ and satisfies the equations (4.1) and (4.2). In the proof the Faedo-Galerkin method is employed.

One investigates the existence of an $\omega$-periodic solution of the problem $u_t=u_{xx}+cu+g(t,x)+ϵf(t,x,u,u_x,ϵ),u(t,0)=h_0\left(t\right)+ϵ{\chi }_0(t,u(t,0),u(t,\pi )),u(t,\pi )=h_1\left(t\right)+ϵ{\chi }_1(t,u(t,0),u(t,\pi ))$, provided the functions $g,f,h_0,h_1,{\chi }_0,{\chi }_1$ are sufficiently smooth and $\omega$-periodic in $t$. If $c\ne k^2$, $k$ natural, such a solution always exists for sufficiently small $ϵ>0$. On the other hand, if $c=l^2$, $l$ natural, some additional conditions have to be satisfied.

In this paper we prove the existence of infinitely many solutions of the Dirac-Fock equations with $N$ electrons turning around a nucleus of atomic charge $Z$, satisfying $N\&lt;Z+1$ and $\alpha max(Z,N)\&lt;2/(2/\pi +\pi /2)$, where $\alpha$ is the fundamental constant of the electromagnetic interaction (approximately 1/137). This work is an improvement of an article of Esteban-Séré, where the same result was proved under more restrictive assumptions on $N$.