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| − | =Problem=
| + | Follow the link! (Original Solution Sheldon) |
| − | Show, by using the Fourier series, that
| + | [https://polybox.ethz.ch/public.php?service=files&t=3cd27159ac0fca5b53c53fe4cff98dcc] |
| − | $$\sum_{k=1}^\infty \frac{1}{k^n} = - \frac{(2 \pi i)^n}{2(n!)} B_n,\ n \in 2 \mathbb{Z}_{>0}$$
| + | |
| − | where \(B_n\) are the Bernoulli numbers defined as
| + | |
| − | $$\frac{z}{e^z - 1} = \sum_{n=0}^\infty B_n \frac{z^n}{n!}$$
| + | |
| | | | |
| − | ''Hint:'' consider \( B_n(x) := \sum_{k \in \mathbb{Z} \setminus \{0\}} k^{-n} e^{ikx},\ n \in \mathbb{Z}_{\geq 2},\ x \in \mathbb{R} \)
| + | An alternative solution can be found here (3.). (MuLö Linalg) |
| | + | [http://www.math.ethz.ch/education/bachelor/lectures/hs2013/math/linalg1/l12] |
| | | | |
| | + | == Task == |
| | + | Determine the center of the algebra of complex d × d-matrices. |
| | + | Hint: the center of an algebra \(A\) is defined as \(Z(A) = \{\phi \in A | \phi \psi = \psi \phi \; \forall \psi \in A \} \). |
| | | | |
| − | =Solution Sketch= | + | == Problem 5 (Sheldon) == |
| − | The solution below is extremely comprehensive, if not expletive, because I felt that the [http://www.math.ethz.ch/~gruppe5/group5/group5.php?lectures/mmp/hs11/ml03.pdf Musterlösung] of the MMP Series that treated the problem was lacking some fairly important steps/guidelines. Therefore find a '''TL;DR''' here:
| + | Let big bold letters denote elements of \(\mathbb{C}^{d \times d}\) and small Greek letters complex numbers. |
| | | | |
| − | # See that \(B_n(0) = 2 \sum_{k=1}^\infty k^{-n}\). We now want to find the \(B_n(0)\) for \(n \in 2\mathbb{Z}_{>0}\)
| + | Let \(M \in Z\) |
| − | # We find a polynomial with Fourier coefficients \(c_0=0\) and \(c_k = \frac{1}{k^2}\). <br/> On \([0,2\pi]\), this polynomial will thus be equal to \(B_2(x)\)
| + | |
| − | # \(B'_n(x) = iB_{n-1}(x)\), therefore all \(B_n(x)\) will be polynomials, too. We find a recursion relation for \(B_n(x)\)
| + | |
| − | # Using the \(2\pi\)-periodicity of \(B_n(x)\) (for all \(n \geq 2\)), we find a recursion relation for the \(B_n(0)\)
| + | |
| − | # We prove the postulated identity by relating the definition for the \(B_n\) given in the problem to the recursion for the \(B_n(0)\) found above.
| + | |
| | | | |
| | + | We make the following observations and conclude that \( Z = \{\lambda\mathbb{I}\}\) |
| | | | |
| − | =Solution=
| |
| − | As suggested in the problem, we will have a look at the convergent fourier series
| |
| − | $$B_n(x) := \sum_{k \in \mathbb{Z} \setminus \{0\}} k^{-n} e^{ikx},\ n \in \mathbb{Z}_{\geq 2},\ x \in \mathbb{R}$$
| |
| − | with Fourier coefficients
| |
| − | $$c_k = \begin{cases}
| |
| − | 0 & \text{for } k = 0\\
| |
| − | \frac{1}{k^n} & \text{for } k \neq 0
| |
| − | \end{cases}$$
| |
| | | | |
| | + | Obviously, the elementes of the center are closed under scalar multiplication (they even form a vector space over \(\mathbb{C}\)). \(\mathbb{I} \in Z \Rightarrow \{\lambda\mathbb{I}\} \subseteq Z\) |
| | | | |
| − | It is obvious that \( B_n(x) \) is \(2\pi\)-periodic and, by differentiating every term separately, that
| |
| − | $$B'_n(x) = iB_{n-1}(x)$$
| |
| | | | |
| − | Also, for \(n \in 2\mathbb{Z}_{>0}\), we see that
| + | The theorem about Jordan decomposition states that \(M\) is conjugate to a matrix in Jordan normal form \(J\) and \(J = CMC^{-1}=MCC^{-1}=M\). |
| − | $$B_n(0) = \sum_{k \in \mathbb{Z} \setminus \{0\}} k^{-n} e^{ik\,0}
| + | |
| − | = \sum_{k=-\infty}^{-1} k^{-n} + \sum_{k=1}^\infty k^{-n} | + | |
| − | = 2 \sum_{k=1}^\infty k^{-n}
| + | |
| − | \tag{$\ddagger$}$$
| + | |
| − | where the last step follows from the fact that \(n\) is even.
| + | |
| | | | |
| − | <hr style="border:0;border-top:thin dashed #ccc;background:none;"/>
| + | \(M\) is a diagonal matrix. |
| | | | |
| − | We will first show that \( B_2(x) = f(x) := \pi^2/3 - \pi x + x^2/2 \) for \( x \in [0, 2\pi]\).
| + | Suppose to the contrary that \(M\) has a '1' in the superdiagonal, then \(2M\) is supposed to be in the center as well, but it can't, because it is not in Jordan normal form. |
| | | | |
| − | Recall the definition of the Fourier coefficients for \(2\pi\)-periodic functions in \(C^\infty(\mathbb{R})\):
| |
| − | $$c_k = \frac{1}{2\pi} \int_0^{2\pi} f(x) \: e^{-ikx} \: dx$$
| |
| | | | |
| − | We can easily verify the following identities:
| + | All diagonal entries of \(M\) are equal. |
| − | $$\begin{align}
| + | |
| − | \frac{1}{2\pi} \int_0^{2\pi} e^{-ikx} \: dx &\ =\
| + | |
| − | \begin{cases}
| + | |
| − | 1 & \text{for } k = 0\\
| + | |
| − | 0 & \text{for } k \neq 0
| + | |
| − | \end{cases} \\
| + | |
| − | \frac{1}{2\pi} \int_0^{2\pi} x \: e^{-ikx} \: dx &\ =\
| + | |
| − | \begin{cases}
| + | |
| − | \pi & \text{for } k = 0\\
| + | |
| − | \frac{i}{k} & \text{for } k \neq 0
| + | |
| − | \end{cases} \\
| + | |
| − | \frac{1}{2\pi} \int_0^{2\pi} x^2 \: e^{-ikx} \: dx &\ =\
| + | |
| − | \begin{cases}
| + | |
| − | \frac{4}{3} \pi^2 & \text{for } k = 0\\
| + | |
| − | \frac{2(1+ik\pi)}{k^2} & \text{for } k \neq 0
| + | |
| − | \end{cases}
| + | |
| − | \end{align}$$
| + | |
| | | | |
| − | Calculating the Fourier coefficients for \(f(x)\) then yields
| + | Let \( M =\begin{pmatrix} \lambda_1 & 0 & \ldots & 0 \\ 0 & \lambda_2 & \ldots & 0 \\ \vdots & &\ddots& \\ 0 & \ldots & 0 & \lambda_d \end{pmatrix} \) and \( E_{ij} \) be the matrix with zero entries except for a '1' in the i-th row and j-th column. |
| − | $$\begin{align}
| + | |
| − | \hat f(k) &= \frac{1}{2\pi} \int_0^{2\pi} f(x) \: e^{-ikx} \: dx\\ | + | |
| − | &= \frac{1}{2\pi} \left( | + | |
| − | \frac{\pi^2}{3} \int_0^{2\pi} e^{-ikx} \: dx
| + | |
| − | - \pi \int_0^{2\pi} x \: e^{-ikx} \: dx
| + | |
| − | + \frac{1}{2} \int_0^{2\pi} x^2 \: e^{-ikx}
| + | |
| − | \right) \\
| + | |
| − | &= \begin{cases} | + | |
| − | 0 & \text{for } k = 0\\
| + | |
| − | \frac{1}{k^2} & \text{for } k \neq 0
| + | |
| − | \end{cases} | + | |
| − | \end{align}$$ | + | |
| − | which proves that \(B_2(x) = f(x)\) for \(x \in [0, 2\pi]\).
| + | |
| | | | |
| − | Thus, we have shown that the \(B_n|_{[0, 2\pi]}\) are polynomials, which are recursively defined by \(B'_n = iB_{n-1}\) and \(\int_0^{2\pi} B_n(x) dx = -i(B_{n+1}(2\pi) - B_{n+1}(0)) = 0\). We also add
| + | \(\lambda_i E_{ij} = ME_{ij} = E_{ij}M = \lambda_j E_{ij} \Rightarrow \lambda_i = \lambda_j \) |
| − | $$B_1(x) = -iB'_2(x) = i\pi - ix \qquad B_0(x) = -iB'_1(x) = -1$$
| + | |
| | | | |
| − | <hr style="border:0;border-top:thin dashed #ccc;background:none;"/>
| + | Because \( 1 \leq i \leq j \leq d \) can be chosen arbitrary, this proves the other inclusion \(M \in \{\lambda\mathbb{I}\}\). |
| | | | |
| − | By induction over \(n\), we will now prove that
| + | == Problem 5 (Musterloesung Illmanen) == |
| − | $$B_n(x) = \sum_{k=0}^n B_{n-k}(0) \frac{(ix)^k}{k!}$$
| + | Let \(E_{ij} \in \mathbb{K}^{n\times n} \) be the matrix which has the entry 1 at \((i,j)\) and 0 else. From proposition \(AE_{ij} = E_{ij}A ~~ \forall 1 \leq i, j \leq n\). Let \(1 \leq i, j \leq n \) be fixed: \(AE_{ij} \) consists solely out of zeros except for the j-th column which is equal to \((a_{1i}, \ldots , a_{ni})^T\). So we get \( a_{ii} = a_{jj} ~~ \wedge ~~ a_{ki} = 0 ~~\forall k \neq j\) |
| − | Base case for \(n=0\):
| + | |
| − | $$B_0(0) \frac{(ix)^0}{0!} = B_0(0) = -1 = B_0(x)$$
| + | |
| − | Inductive step (\(n\!-\!1 \rightarrow n\)):
| + | |
| − | $$\begin{align}
| + | |
| − | B_n(x) &= \int_0^x iB_{n-1}(\tilde x)\ d\tilde x + B_n(0) \\
| + | |
| − | &= \int_0^x i\left(\sum_{k=0}^{n-1} B_{(n-1)-k}(0) \frac{(i\tilde x)^k}{k!} \right) d\tilde x + B_n(0) \\
| + | |
| − | &= \left( \sum_{k=0}^{n-1} B_{(n-1)-k}(0) \frac{i^{k+1}}{k!} \int_0^x \tilde x^k\ d\tilde x \right) + B_n(0) \\
| + | |
| − | &= \left( \sum_{k=0}^{n-1} B_{(n-1)-k}(0) \frac{i^{k+1}}{k!} \frac{x^{k+1}}{k+1} \right) + B_n(0) \\
| + | |
| − | &= \left( \sum_{k=0}^{n-1} B_{n-(k+1)}(0) \frac{(ix)^{k+1}}{(k+1)!} \right) + B_n(0) \\
| + | |
| − | &= \left( \sum_{k=1}^n B_{n-k}(0) \frac{(ix)^k}{k!} \right) + B_n(0) \\
| + | |
| − | &= \sum_{k=0}^n B_{n-k}(0) \frac{(ix)^k}{k!}
| + | |
| − | \end{align}$$
| + | |
| | | | |
| − | <hr style="border:0;border-top:thin dashed #ccc;background:none;"/>
| + | We can do this for all indices and this leads to \(a_{ij} = 0 \forall i \neq j ~~\wedge ~~ a_{11} = a_{22} = \ldots = a_{nn} \). Therefor \(A\) has to have the form \(\lambda E_n, \lambda \in \mathbb{K}\) |
| | | | |
| − | As \(B_n\) is \(2\pi\)-periodic for all \(n \geq 2\), the above identity leads us to the relation
| + | ===Ilmanen's solution componentwise=== |
| − | $$0 = B_n(2\pi) - B_n(0) = \sum_{k=1}^n B_{n-k}(0) \frac{(2\pi i)^k}{k!} \tag{*}$$
| + | |
| − | for all \(n \geq 2\), which determines all \(B_n(0)\) from \(B_0(0) = -1\)
| + | |
| | | | |
| − | <hr style="border:0;border-top:thin dashed #ccc;background:none;"/>
| + | Let \(A \in Z(\mathrm{Mat}_d(\mathbb{C}))\). |
| | | | |
| − | The definition of the \(B_n\) in the problem (the Bernoulli numbers, not the functions!) can be transformed to:
| + | Let \(E_{ij}\) be the \(d \times d\)-matrix with \((E_{ij})_{kl} = \delta_{ik} \delta_{jl} \). |
| − | $$\begin{align}
| + | |
| − | & \frac{z}{e^z - 1} = \sum_{n=0}^\infty B_n \frac{z^n}{n!} \\
| + | |
| − | \Leftrightarrow\ & z = \left( \sum_{n=0}^\infty B_n \frac{z^n}{n!} \right) \left( \sum_{k=1}^\infty \frac{z^k}{k!} \right) \\
| + | |
| − | \Leftrightarrow\ & z = \left( B_0 + B_1z + \frac{B_2}{2}z^2 + \frac{B_3}{3!}z^3 + ... \right) \left( z + \frac{z^2}{2} + \frac{z^3}{3!} + \frac{z^4}{4!} + ... \right) \\
| + | |
| − | \Leftrightarrow\ & z = B_0z + \left( \frac{B_0}{2!} + B_1 \right)z^2 + \left( \frac{B_0}{3!} + \frac{B_1}{2!} + \frac{B_2}{2!} \right)z^3 + \left( \frac{B_0}{4!} + \frac{B_1}{3!} + \frac{B_2}{2!2!} + \frac{B_3}{3!} \right)z^4
| + | |
| − | \end{align}$$
| + | |
| − | which, through equating coefficients yields \(B_0 = 1\) and for \(n \geq 2\)
| + | |
| − | $$\sum_{k=1}^n \frac{B_{n-k}}{(n-k)!k!} = 0 \tag{$\dagger$}$$
| + | |
| | | | |
| − | <hr style="border:0;border-top:thin dashed #ccc;background:none;"/>
| + | Now let \(1 \leq i, j \leq d\) with \(i \neq j\) be fixed and consider |
| | + | $$(E_{ij} A)_{kl} = \sum \limits_{m}(E_{ij})_{km} A_{ml} = \sum \limits_{m}\delta_{ki} \delta_{jm} A_{ml} = \delta_{ki} A_{jl}.$$ |
| | + | Since \(A\) commutes with all compex \(d \times d\)-matrices, this is the same as |
| | + | $$(A E_{ij})_{kl} = \sum \limits_{m}A_{km} (E_{ij})_{ml} = \sum \limits_{m} A_{km} \delta_{mi} \delta_{jl} = \delta_{jl} A_{ki}.$$ |
| | | | |
| − | <span style="color:#339;font-size:80%;">The following proof by induction doesn't appear in the aforementioned Musterlösung, where the final step simply falls out of thin air. It is not unlikely, however, that I simply don't understand what they do. If this is the case, feel free to simplify the following mess of equations. --Nik</span>
| + | Thus, we have that |
| − | | + | $$\delta_{ki} A_{jl} = \delta_{jl} A_{ki}$$ |
| − | We now now prove by induction that for \(n \geq 0\)
| + | Now set \(k=i, l=j\). We then find |
| − | $$B_n(0) = - \frac{(2\pi i)^n B_n}{n!}$$ | + | $$A_{ii} = A_{jj}$$ |
| − | | + | Next, set \(k=l=i\). Still, \(j \neq i\). It follows that |
| − | Base case for \(n=0\):
| + | $$A_{ji} = 0.$$ |
| − | $$B_0(0) = -1 = -B_0$$
| + | As this holds for an arbitrary choice of \(i \neq j\), the required form for \(A\) is |
| − | Inductive step (\(n\!-\!1 \rightarrow n\)):
| + | $$A = \lambda \mathbb{I}_d$$ |
| − | $$\begin{align}
| + | for some \(\lambda \in \mathbb{C}\). And obviously \(\forall \lambda \in \mathbb{C}: \lambda \mathbb{I} \in Z(\mathrm{Mat}_d(\mathbb{C}))\). |
| − | \sum_{k=1}^{n+1} B_{n+1-k}(0) \frac{(2\pi i)^k}{k!} \ &\overset{(*)}{=} \ 0 \\
| + | |
| − | \Leftrightarrow \ B_n(0)\: 2\pi i \ &= \: - \sum_{k=2}^{n+1} B_{n+1-k}(0) \frac{(2\pi i)^k}{k!} \\ | + | |
| − | \overset{\text{Hyp.}}{\Leftrightarrow} \ B_n(0)\: 2\pi i \ &= \: - \sum_{k=2}^{n+1} \left( - \frac{(2\pi i)^{n+1-k} B_{n+1-k}}{(n+1-k)!} \right) \frac{(2\pi i)^k}{k!} \\
| + | |
| − | \Leftrightarrow \ B_n(0)\: 2\pi i \ &= \ (2\pi i)^{n+1} \sum_{k=2}^{n+1} \frac{B_{n+1-k}}{(n+1-k)!\:k!} \\
| + | |
| − | \overset{(\dagger)}{\Leftrightarrow} \ B_n(0)\: 2\pi i \ &= \ (2\pi i)^{n+1} \left(- \frac{B_n}{n!} \right) \\
| + | |
| − | B_n(0) \ &= \: - \frac{(2\pi i)^n B_n}{n!}
| + | |
| − | \end{align}$$
| + | |
| − | | + | |
| − | This, together with equation \((\ddagger)\), proves the identity in the problem.
| + | |
| − | | + | |
| − | <p style="text-align:right;">\(\square\)</p>
| + | |
| − | | + | |
| − | | + | |
| − | % =Alternate Solution=
| + | |
| − | % Let $\Zeta(s) = \sum\limits_{n=0}^\infty \frac{1}{n^s}$ for $\left{s \in \mathbb{C}\mid \Re (s) > 1 \right}$ \\
| + | |
| − | % Consider
| + | |
| − | % $$ E(z) = \frac{z}{e^z-1} = \sum\limits_{n=0}^\infty B_n \frac{z^n}{n!} \quad\quad\quad z \in \mathbb(R)$$
| + | |
| − | | + | |
| − | | + | |
| − | % =Alternate Solution=
| + | |
| − | | + | |
| − | Let $$\zeta(s) = \sum\limits_{n=1}^\infty \frac{1}{n^s} $$ for $$s \in \mathbb{C}, \Re (s) > 1$$
| + | |
| − | | + | |
| − | Consider
| + | |
| − | $$ E(z) := \frac{z}{e^z -1} := \sum\limits_{n=0}^\infty B_n \frac{z^n}{n!} \quad\quad\quad z \in \mathbb{R} $$
| + | |
| − | $$\lim\limits_{z \to 0}{E(z)} = \lim\limits_{z \to 0}{\frac{1}{e^z}} = 1 = \lim\limits_{z \to 0}{\sum\limits_{n=0}^\infty B_n \frac{z^n}{n!}} $$
| + | |
| − | Where in the first equality we used l'Hospital's rule and with
| + | |
| − | $$ \lim\limits_{z \to 0}{E(z)} = E(\lim\limits_{z \to 0}{z}) = \sum\limits_{n=0}^\infty \lim\limits_{z \to 0}{B_n \frac{z^n}{n!}} =B_0 $$
| + | |
| − | Where we can put the limit inside because E is continuous and bounded in a neighborhood of $$z_0 = 0$$.\\
| + | |
| − | $$ \rightarrow B_0 = 1 $$ | + | |
| − | Now we repeat the same thing for $$E'$$ \\
| + | |
| − | \begin{align*}
| + | |
| − | \lim\limits_{z \to 0}{E'(z)} & = \lim\limits_{z \to 0}{\frac{e^z -1 -z e^z}{e^{2z} -2e^z+1}} \\
| + | |
| − | & = \lim\limits_{z \to 0}{\frac{e^z -z e^z -e^z}{2e^{2z} -2e^z}} \\
| + | |
| − | & = \lim\limits_{z \to 0}{\frac{-z e^z - e^z}{4e^{2z}-2e^z}} \\
| + | |
| − | & = -\frac{1}{2}
| + | |
| − | \end{align*}
| + | |
| − | Here we again used l'Hospital's rule for the second and third equality. For the same reasons as above we get
| + | |
| − | $$ \lim\limits_{z \to 0}{E'(z)} = B_1 $$
| + | |
| − | and therefore
| + | |
| − | $$\rightarrow b_1 = - \frac{1}{2}$$
| + | |
| − | | + | |
| − | $$\Rightarrow E(z) = 1 - \frac{1}{2} z + \mathcal{O}(z^2)$$
| + | |
| − | | + | |
| − | Now define $$f(z) = \frac{z}{2} \coth (\frac{z}{2})$$ on $$]-\frac{1}{2}, \frac{1}{2}]$$\\
| + | |
| − | \begin{align*}
| + | |
| − | f(z) & = \frac{z}{2} \coth (\frac{z}{2}) \\
| + | |
| − | & = \frac{z e^{\frac{z}{2}} + z e^{-\frac{z}{2}}}{2e^{\frac{z}{2}} - 2 e^{-\frac{z}{2}}} \\
| + | |
| − | & = \frac{z e^{\frac{z}{2}} + z e^{-\frac{z}{2}}}{2e^{\frac{z}{2}}(e^z-1)} \\
| + | |
| − | & = \frac{z}{2} \frac{e^z+1}{e^z-1} \\
| + | |
| − | & = \frac{z}{2} \left( 1 + \frac{2}{e^z-1} \right) \\
| + | |
| − | & = \frac{z}{2} + \frac{z}{e^z-1} \\
| + | |
| − | & = \frac{z}{2} + E(z)
| + | |
| − | \end{align*}
| + | |
| − | | + | |
| − | Since $$z$$ and $$\coth$$ are odd functions, $$f(z)$$ is an even function. From the part above we know that $$E(z) = 1 - \frac{1}{2} z + \mathcal{O}(z^2)$$. Since the addition of $$\frac{z}{2}$$ kills the uneven term and $$f(z)$$ has to be even, the only remaining coefficients are even:\\
| + | |
| − | $$ f(z) = \frac{z}{2} + E(z) = \sum\limits_{n=0}^\infty B_{2n} \frac{z^{2n}}{(2n)!} $$ | + | |
| − | | + | |
| − | Now define $$g(x) = \cosh (zx)$$ on $$]-\frac{1}{2}, \frac{1}{2}]$$
| + | |
| − | with $$z \in \mathbb{R} \setminus \{0\}$$, continued periodically on $$\mathbb{R}$$. \\
| + | |
| − | Calculating the Fourier coefficients of $$g$$: \\
| + | |
| − | | + | |
| − | $$\alpha_0 = \int\limits_{-\frac{1}{2}} ^{\frac{1}{2}} \cosh(zx)dx = \frac{2}{z}\sinh (\frac{z}{2})$$
| + | |
| − | | + | |
| − | \begin{align*}
| + | |
| − | \alpha_n & = \int\limits_{-\frac{1}{2}} ^{\frac{1}{2}} \cosh(zx)\cos(2\pi n x)dx \\
| + | |
| − | & = \left[\cosh(zx)\frac{1}{2\pi n}\sin(2\pi nx) \right]_{-\frac{1}{2}}^{\frac{1}{2}} - \int\limits_{-\frac{1}{2}} ^{\frac{1}{2}} z \sinh (zx) \frac{1}{2\pi n} \sin (2\pi nx) dx \\
| + | |
| − | & = -z \int\limits_{-\frac{1}{2}} ^{\frac{1}{2}}\sinh (zx) \frac{1}{2\pi n} \sin (2\pi nx) dx \\
| + | |
| − | & = -z \left[ \left[-\sinh (zx) \left(\frac{1}{2\pi n}\right)^2 \cos (2\pi nx) \right]_{-\frac{1}{2}}^{\frac{1}{2}} + z \int\limits_{-\frac{1}{2}}^{\frac{1}{2}} \cosh (zx)\left(\frac{1}{2\pi n}\right)^2 \cos (2\pi nx) dx \right]
| + | |
| − | \end{align*}
| + | |
| − | | + | |
| − | $$ \Rightarrow \left(1 + \left(\frac{z}{2\pi n}\right)^2\right)\int\limits_{-\frac{1}{2}}^{\frac{1}{2}} \cosh (zx) \cos (2\pi nx) dx = 2z\left(\frac{z}{2\pi n}\right)^2 (-1)^n \sinh (\frac{z}{2}) $$
| + | |
| − | | + | |
| − | $$ \rightarrow \alpha_n = \int\limits_{-\frac{1}{2}}^{\frac{1}{2}} \cosh (zx) \cos (2\pi nx) dx = \frac{2z}{(2\pi n)^2 + z^2} (-1)^n \sinh (\frac{z}{2}) $$
| + | |
| − | Where we use $$\cos(2\pi n x)$$ because the integrand is even. Therefore we get for the Fourieseries of $$g$$:\\
| + | |
| − | | + | |
| − | $$ \cosh (zx) = \frac{2}{z} \sinh (\frac{z}{2}) \left[ 1 + \sum\limits_{n \in \mathbb{Z} \\ n \neq 0} (-1)^n \frac{2z}{4{\pi}^2 n^2 + z^2} \cos (2\pi n x) \right] $$
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| − | $$ = \frac{2}{z} \sinh(\frac{z}{2}) \left[ 1 + 2\sum\limits_{n=1}^\infty (-1)^n \frac{2z}{4{\pi}^2 n^2 + z^2} \cos (2\pi n x) \right]$$
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| − | | + | |
| − | Now we chose $$x = \frac{1}{2}$$:
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| − | | + | |
| − | $$ \cosh (\frac{z}{2}) = \frac{2}{z} \sinh(\frac{z}{2}) \left[ 1 + 2\sum\limits_{n=1}^\infty (-1)^n \frac{z^2}{4\pi^2 n^2} \frac{1}{1+\frac{z^2}{4\pi^2 n^2}} \right] $$
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| − | | + | |
| − | $$ \underbrace{\frac{z}{2} \frac{\cosh (\frac{z}{2})}{\sinh(\frac{z}{2})}}_{f(z)}
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| − | = 1 + 2\sum\limits_{n=1}^\infty (-1)^n \frac{z^2}{4{\pi}^2 n^2} \frac{1}{1+\frac{z^2}{4{\pi}^2 n^2}} $$
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| − | $$ = 1 + \sum\limits_{n=1}^\infty \sum\limits_{k=0}^\infty \frac{2z^2}{4{\pi}^2 n^2} \left(-\frac{z^2}{4\pi^2 n^2}\right)^k$$
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| − | | + | |
| − | Because both series are absolute convergent and uniform convergent we get after some calculation:
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| − | $$ f(z) = 1 + \sum\limits_{k = 1}^\infty 2 (-1)^{k-1} \zeta(2k) \frac{z^{2k}}{(2\pi)^{2k}} $$
| + | |
| − | but above we also calculated
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| − | | + | |
| − | $$ f(z) =1 + \sum\limits_{n=1}^\infty B_{2n} \frac{z^{2n}}{(2n)!} $$
| + | |
| − | With a coefficient comparison we get:
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| − | | + | |
| − | $$ \zeta(2n) = - \frac{B_{2n}(-1)^{2n}(2\pi)^{2n}}{2(2n)!} = -\frac{(2\pi i)^{2n}}{2(2n)!}B_{2n} \quad\quad\quad \forall n \in \mathbb{N} $$
| + | |
| − | which is what we had to show.
| + | |
Determine the center of the algebra of complex d × d-matrices.
Hint: the center of an algebra \(A\) is defined as \(Z(A) = \{\phi \in A | \phi \psi = \psi \phi \; \forall \psi \in A \} \).
Let big bold letters denote elements of \(\mathbb{C}^{d \times d}\) and small Greek letters complex numbers.
We make the following observations and conclude that \( Z = \{\lambda\mathbb{I}\}\)
Suppose to the contrary that \(M\) has a '1' in the superdiagonal, then \(2M\) is supposed to be in the center as well, but it can't, because it is not in Jordan normal form.
Let \( M =\begin{pmatrix} \lambda_1 & 0 & \ldots & 0 \\ 0 & \lambda_2 & \ldots & 0 \\ \vdots & &\ddots& \\ 0 & \ldots & 0 & \lambda_d \end{pmatrix} \) and \( E_{ij} \) be the matrix with zero entries except for a '1' in the i-th row and j-th column.
\(\lambda_i E_{ij} = ME_{ij} = E_{ij}M = \lambda_j E_{ij} \Rightarrow \lambda_i = \lambda_j \)
Because \( 1 \leq i \leq j \leq d \) can be chosen arbitrary, this proves the other inclusion \(M \in \{\lambda\mathbb{I}\}\).
Let \(E_{ij} \in \mathbb{K}^{n\times n} \) be the matrix which has the entry 1 at \((i,j)\) and 0 else. From proposition \(AE_{ij} = E_{ij}A ~~ \forall 1 \leq i, j \leq n\). Let \(1 \leq i, j \leq n \) be fixed: \(AE_{ij} \) consists solely out of zeros except for the j-th column which is equal to \((a_{1i}, \ldots , a_{ni})^T\). So we get \( a_{ii} = a_{jj} ~~ \wedge ~~ a_{ki} = 0 ~~\forall k \neq j\)
We can do this for all indices and this leads to \(a_{ij} = 0 \forall i \neq j ~~\wedge ~~ a_{11} = a_{22} = \ldots = a_{nn} \). Therefor \(A\) has to have the form \(\lambda E_n, \lambda \in \mathbb{K}\)
Let \(E_{ij}\) be the \(d \times d\)-matrix with \((E_{ij})_{kl} = \delta_{ik} \delta_{jl} \).
Now let \(1 \leq i, j \leq d\) with \(i \neq j\) be fixed and consider
$$(E_{ij} A)_{kl} = \sum \limits_{m}(E_{ij})_{km} A_{ml} = \sum \limits_{m}\delta_{ki} \delta_{jm} A_{ml} = \delta_{ki} A_{jl}.$$
Since \(A\) commutes with all compex \(d \times d\)-matrices, this is the same as
$$(A E_{ij})_{kl} = \sum \limits_{m}A_{km} (E_{ij})_{ml} = \sum \limits_{m} A_{km} \delta_{mi} \delta_{jl} = \delta_{jl} A_{ki}.$$
Thus, we have that
$$\delta_{ki} A_{jl} = \delta_{jl} A_{ki}$$
Now set \(k=i, l=j\). We then find
$$A_{ii} = A_{jj}$$
Next, set \(k=l=i\). Still, \(j \neq i\). It follows that
$$A_{ji} = 0.$$
As this holds for an arbitrary choice of \(i \neq j\), the required form for \(A\) is
$$A = \lambda \mathbb{I}_d$$
for some \(\lambda \in \mathbb{C}\). And obviously \(\forall \lambda \in \mathbb{C}: \lambda \mathbb{I} \in Z(\mathrm{Mat}_d(\mathbb{C}))\).
