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# 数学代写|数学分析作业代写Mathematical Analysis代考|MATH2400 The Spectrum of an Operator

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## 数学代写|数学分析代写MATHEMATICAL ANALYSIS代考|The Spectrum of an Operator

The spectrum of a square matrix $A$ is simply its set of eigenvalues, and the eigenvalues of $A$ are easy to characterize. They are exactly the complex numbers $\lambda$ for which the matrix $A-\lambda I$ is not invertible. We recall the simple fact that $A-\lambda I$ is not invertible if and only if the linear operator $T$ it generates on $\mathbb{K}^n$ is not oneto-one, and this is the case if and only if $T$ in not onto.

The definition of the spectrum of an operator $T$ on an infinite-dimensional space is exactly the same as it is for a matrix. The stark distinction here is that not every point in the spectrum of an operator on an infinite-dimensional space is an eigenvalue. This is because such an operator may be one-to-one but not onto or conversely. See example 1 . Thus the spectrum consists of two main parts: the complex numbers $\lambda$ for which $T-\lambda I$ is not one-to-one (the eigenvalues) and those for which $T-\lambda I$ is one to one but not onto. The spectrum of an operator $T$ often carries valuable information about $T$, and, in some cases, the eigenvalues of an operator and the corresponding eigenvectors completely define the operator.
Definition. A Banach algebra is a Banach space $X$ that is also an algebra with a multiplicative identity $I$ such that the norm satisfies the following additional assumptions:
(a) $|I|=1$, and
(b) $|S T| \leq|S| T |$ for all $S$ and $T$ in $X$.

## 数学代写|数学分析代写MATHEMATICAL ANALYSIS代考|Adjoint Operators and Quotient Spaces

In section 3.7, we defined the adjoint of an operator on a finite-dimensional inner product space, and, in chapter 7, we will study adjoints of operators on a Hilbert space. The definition of the adjoints on Banach spaces $X$ is more complicated. In fact, the adjoint of a bounded operator on a Banach space $X$ is a bounded operator on the dual space $X^$. Among other results, we prove that an operator $T$ and its adjoint, $T^$ have the same norm, the same spectrum, and the same spectral radius. We also study annihilators and quotient spaces. Little subsequent material rests on this section, and it is possible to study the remainder of the book independently of this section.

Notation. The duality bracket: Let $X$ be a Banach space. For $x \in X$ and $\lambda \in X^*$, we write $\langle x, \lambda\rangle$ for $\lambda(x)$. This is a notational convenience that also facilitates certain computations. In addition, the notation equalizes the roles of $X$ and $X^$. We already saw that $X$ acts on $X^$ in much the same way $X^*$ acts on $X$. See, for example, the construction leading up to theorem 6.4.8. Observe that $|\langle x, \lambda\rangle| \leq|x||\lambda|$, reminiscent of the Cauchy-Schwarz inequality. We revert to the traditional notation $\lambda(x)$ when convenient.

## 数学代写|数学分析代写MATHEMATICAL ANALYSIS代考|The Spectrum of an Operator

(a) $|I|=1$, 和
(b) $|S T| \leq|S| T \mid$ 对所有人 $S$ 和 $T$ 在 $X$.

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