Let $f:B(x_0,R)\subseteq X \rightarrow Y$ be an operator, with $X$ and $Y$ Banach spaces, and $f'$ be H\"{o}lder continuous with exponent $\theta$. The convergence of the sequence of Newton-Kantorovich approximations $$x_n=x_{n-1} -f'(x_{n-1})^{-1}f(x_{n-1}) \, \quad n \in N$$ is a classical tool to solve the equation $f(x)=0$.\\ The convergence of $x_n$ is often reduced to the study of the majorizing sequence $r_n$ defined by $$r_0=0, \quad r_1 =a , \quad r_{n+1}=r_n +\frac{bk(r_n-r_{n-1})^{1+\theta}}{(1+\theta)(1-bkr_n ^{\theta})} \,,$$ with $a, b, k$ parameters related to $f$ and $f'$.\\ We extend an estimate for $r_n$, known in the Lipschitz case, to the H\"{o}lder case. The proof requires the introduction of a multiplicative factor in the sequence estimating $r_n$, estimates of the ratio $\displaystyle{\frac{r_{n+1}}{r_n}}$ and the use of two parallel induction processes on the sequences $r_n$ and $\displaystyle{\frac{r_{n+1}}{r_n}}$.\\ In the last part we make a comparison with our previous results.

### Estimates of majorizing sequences in the Newton-Kantorovich method

#### Abstract

Let $f:B(x_0,R)\subseteq X \rightarrow Y$ be an operator, with $X$ and $Y$ Banach spaces, and $f'$ be H\"{o}lder continuous with exponent $\theta$. The convergence of the sequence of Newton-Kantorovich approximations $$x_n=x_{n-1} -f'(x_{n-1})^{-1}f(x_{n-1}) \, \quad n \in N$$ is a classical tool to solve the equation $f(x)=0$.\\ The convergence of $x_n$ is often reduced to the study of the majorizing sequence $r_n$ defined by $$r_0=0, \quad r_1 =a , \quad r_{n+1}=r_n +\frac{bk(r_n-r_{n-1})^{1+\theta}}{(1+\theta)(1-bkr_n ^{\theta})} \,,$$ with $a, b, k$ parameters related to $f$ and $f'$.\\ We extend an estimate for $r_n$, known in the Lipschitz case, to the H\"{o}lder case. The proof requires the introduction of a multiplicative factor in the sequence estimating $r_n$, estimates of the ratio $\displaystyle{\frac{r_{n+1}}{r_n}}$ and the use of two parallel induction processes on the sequences $r_n$ and $\displaystyle{\frac{r_{n+1}}{r_n}}$.\\ In the last part we make a comparison with our previous results.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11770/143078
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