  In this paper, we consider a problem in calculus of variations motivated by a
quantitative isoperimetric inequality in the plane. More precisely, the aim of
this article is the computation of the minimum of the variational problem
$$\inf_{u\in\mathcal{W}}\frac{\displaystyle\int_{-\pi}^{\pi}[(u')^2-u^2]d\theta}{\displaystyle\left[\int_{-\pi}^{\pi}|u|
d\theta\right]^2}$$where $u\in \mathcal{W}$ is a $H^1(-\pi,\pi)$ periodic
function, with zero average on $(-\pi,\pi)$ and orthogonal to sine and cosine.
