In this paper we study the quantitative isoperimetric inequality in the plane. We prove the existence of a set $Omega$, different from a ball, which minimizes the ratio $delta(Omega)/lambda^2(Omega)$, where $delta$ is the isoperimetric deficit and $lambda$ the Fraenkel asymmetry, giving a new proof ofthe quantitative isoperimetric inequality. Some new properties of the optimal set are also shown.
Given a positive lower semi-continuous density $f$ on $mathbb{R}^2$ the weighted volume $V_f:=fmathscr{L}^2$ is defined on the $mathscr{L}^2$-measurable sets in $mathbb{R}^2$. The $f$-weighted perimeter of a set of finite perimeter $E$ in $mathbb{R}^2$ is written $P_f(E)$. We study minimisers for the weighted isoperimetric problem [ I_f(v):=infBig{ P_f(E):Etext{ is a set of finite perimeter in }mathbb{R}^2text{ and }V_f(E)=vBig} ] for $v>0$. Suppose $f$ takes the form $f:mathbb{R}^2rightarrow(0,+infty);xmapsto e^{h(|x|)}$ where $h:[0,+infty)rightarrowmathbb{R}$ is a non-decreasing convex function. Let $v>0$ and $B$ a centred ball in $mathbb{R}^2$ with $V_f(B)=v$. We show that $B$ is a minimiser for the above variational problem and obtain a uniqueness result.
We consider the monomial weight $x^{A}=vert x_{1}vert^{a_{1}}ldotsvert x_{N}vert^{a_{N}}$, where $a_{i}$ is a nonnegative real number for each $iin{1,ldots,N}$, and we establish the existence and nonexistence of isoperimetric inequalities with different monomial weights. We study positive minimizers of $int_{partialOmega}x^{A}mathcal{H}^{N-1}(x)$ among all smooth bounded sets $Omega$ in $mathbb{R}^{N}$ with fixed Lebesgue measure with monomial weight $int_{Omega}x^{B}dx$.
We establish the Trudinger-Moser inequality on weighted Sobolev spaces in the whole space, and for a class of quasilinear elliptic operators in radial form of the type $displaystyle Lu:=-r^{-theta}(r^{alpha}vert u(r)vert^{beta}u(r))$, where $theta, betageq 0$ and $alpha>0$, are constants satisfying some existence conditions. It worth emphasizing that these operators generalize the $p$- Laplacian and $k$-Hessian operators in the radial case. Our results involve fractional dimensions, a new weighted Polya-Szeg{o} principle, and a boundness value for the optimal constant in a Gagliardo-Nirenberg type inequality.
} In this article, we put forward a Neumann eigenvalue problem for the bi-harmonic operator $Delta^2$ on a bounded smooth domain $Om$ in the Euclidean $n$-space ${bf R}^n$ ($nge2$) and then prove that the corresponding first non-zero eigenvalue $Upsilon_1(Om)$ admits the isoperimetric inequality of Szego-Weinberger type: $Upsilon_1(Om)le Upsilon_1(B_{Om})$, where $B_{Om}$ is a ball in ${bf R}^n$ with the same volume of $Om$. The isoperimetric inequality of Szego-Weinberger type for the first nonzero Neumann eigenvalue of the even-multi-Laplacian operators $Delta^{2m}$ ($mge1$) on $Om$ is also exploited.