A.G.Ramm

  1. Uniqueness of the solution to inverse scattering problem with backscattering data.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    Let $q(x)$ be real-valued compactly supported sufficiently smooth function.
    It is proved that the scattering data $A(-\beta,\beta,k)$ $\forall \beta\in
    S^2$, $\forall k>0,$ determine $q$ uniquely.

  2. A uniqueness theorem for entire functions.

    Authors: A.G.Ramm
    Subjects: Complex Variables
    Abstract

    Let $G(k)=\int_0^1g(x)e^{kx}dx$, $g\in L^1(0,1)$. The main result of this
    paper is the following theorem.

    {\bf Theorem}. {\it If $\limsup_{k\to +\infty}|G(k)|<\infty$, then $g=0$.
    There exists $g\not\equiv 0$, $g\in L^1(0,1)$, such that $G(k_j)=0$,
    $k_j<k_{j+1}$, $\lim_{j\to \infty}k_j=\infty$, $\lim_{k\to \infty}|G(k)|$ does
    not exist, $\limsup_{k\to +\infty}|G(k)|=\infty$. This $g$ oscillates
    infinitely often in any interval $[1-\delta, 1]$, however small $\delta>0$ is.}

  3. On a new notion of the solution to an ill-posed problem.

    Authors: A.G.Ramm
    Subjects: Numerical Analysis
    Abstract

    A new understanding of the notion of the stable solution to ill-posed
    problems is proposed. The new notion is more realistic than the old one and
    better fits the practical computational needs. A method for constructing stable
    solutions in the new sense is proposed and justified.

  4. Inversion of the Laplace transform from the real axis using an adaptive iterative method.

    Authors: A.G.Ramm, S.W.Indratno
    Subjects: Numerical Analysis
    Abstract

    In this paper a new method for inverting the Laplace transform from the real
    axis is formulated. This method is based on a quadrature formula. We assume
    that the unknown function $f(t)$ is continuous with (known) compact support. An
    adaptive iterative method and an adaptive stopping rule, which yield the
    convergence of the approximate solution to $f(t)$, are proposed in this paper.

  5. Property $C$ and applications to inverse problems.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    Let $\ell_j:=-\frac{d^2}{dx^2}+k^2q_j(x),$ $k=const>0, j=1,2,$ $0<c_0\leq
    q_j(x)\leq c_1,$ %$q\in BV([0,1])$, $q$ has finitely many discontinuity points
    $x_m\in [0,1],$ and is real-analytic on the intervals $[x_m,x_{m+1}]$ between
    these points. The set of such functions $q$ is denoted by $M.$ Only the
    following property of $M$ is used: if $q_j\in M$, $j=1,2,$ then the function
    $p(x):=q_2(x)-q_1(x)$ changes sign on the interval $[0, 1]$ at most finitely
    many times.

  6. Creating materials with a desired refraction coefficient.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    A method is given for creating material with a desired refraction
    coefficient. The method consists of embedding into a material with known
    refraction coefficient many small particles of size $a$. The number of
    particles per unit volume around any point is prescribed, the distance between
    neighboring particles is $O(a^{\frac{2-\kappa}{3}})$ as $a\to 0$, $0<\kappa<1$
    is a fixed parameter. The total number of the embedded particle is
    $O(a^{\kappa-2})$.

  7. Creating materials with a desired refraction coefficient.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    A method is given for creating material with a desired refraction
    coefficient. The method consists of embedding into a material with known
    refraction coefficient many small particles of size $a$. The number of
    particles per unit volume around any point is prescribed, the distance between
    neighboring particles is $O(a^{\frac{2-\kappa}{3}})$ as $a\to 0$, $0<\kappa<1$
    is a fixed parameter. The total number of the embedded particle is
    $O(a^{\kappa-2})$.

  8. Existence of solution to an evolution equation and a justification of the DSM for equations with monotone operators.

    Authors: A.G.Ramm, N.S.Hoang
    Subjects: Mathematical Physics
    Abstract

    An evolution equation, arising in the study of the Dynamical Systems Method
    (DSM) for solving equations with monotone operators, is studied in this paper.
    The evolution equation is a continuous analog of the regularized Newton method
    for solving ill-posed problems with monotone nonlinear operators $F$. Local and
    global existence of the unique solution to this evolution equation are proved,
    apparently for the firs time, under the only assumption that $F'(u)$ exists and
    is continuous with respect to $u$. The earlier published results required more
    smoothness of $F$.

  9. A method for creating materials with a desired refraction coefficient.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    It is proposed to create materials with a desired refraction coefficient in a
    bounded domain $D\subset \R^3$ by embedding many small balls with constant
    refraction coefficients into a given material. The number of small balls per
    unit volume around every point $x\in D$, i.e., their density distribution, is
    calculated, as well as the constant refraction coefficients in these balls.
    Embedding into $D$ small balls with these refraction coefficients according to
    the calculated density distribution creates in $D$ a material with a desired
    refraction coefficient.

  10. Boundary integral equation for electromagnetic wave scattering by a homogeneous body of arbitrary shape.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    Boundary integral equation is derived for the problem of scattering of
    electromagnetic waves by 3D homogeneous body of arbitrary shape.

  11. A singular integral equation for electromagnetic wave scattering.

    Authors: A.G.Ramm
    Subjects: Mathematical Physics
    Abstract

    A 3D singular integral equation is derived for electromagnetic wave
    scattering by bodies of arbitrary shape. Its numerical solution by a projection
    method is outlined.

  12. Hard Implicit Function Theorem via the DSM.

    Authors: A.G.Ramm
    Subjects: Functional Analysis
    Abstract

    Sufficient conditions are given for a hard implicit function theorem to hold.
    The result is established by an application of the Dynamical Systems Method
    (DSM). It allows one to solve a class of nonlinear operator equations in the
    case when the Fr\'echet derivative of the nonlinear operator is a smoothing
    operator, so that its inverse is an unbounded operator.

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