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@@ -29,25 +29,25 @@ |
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\begin{align*} |
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\int_{-1}^{1} P_n(x) P_m(x) \d x |
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&= \frac{1}{2^{n} 2^{m} n! m!} |
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\int_{-1}^{1} \frac{\mathrm{d}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}}{\d x^{m}}(x^2-1)^{m} \d x |
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\int_{-1}^{1} \frac{\mathrm{d}^{n}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}^{m}}{\d x^{m}}(x^2-1)^{m} \d x |
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.\end{align*} |
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Zu zeigen: |
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\[ |
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\int_{-1}^{1} \frac{\mathrm{d}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}}{\d x^{m}}(x^2-1)^{m} \d x = 0 |
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\int_{-1}^{1} \frac{\mathrm{d}^{n}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}^{m}}{\d x^{m}}(x^2-1)^{m} \d x = 0 |
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.\] Mit $(*)$ und $k = m+1 \le n$ folgt |
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\begin{align*} |
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\int_{-1}^{1} \frac{\mathrm{d}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}}{\d x^{m}}(x^2-1)^{m} \d x |
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\int_{-1}^{1} \frac{\mathrm{d}^{n}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}^{m}}{\d x^{m}}(x^2-1)^{m} \d x |
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&= (-1)^{m+1} \int_{-1}^{1} \frac{\mathrm{d}^{n-m-1}}{\d x^{n-m-1}} |
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(x^2-1)^{n} \frac{\mathrm{d}^{2m+1}}{\d x^{2m+1}}(x^2-1)^{m}\d x |
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.\end{align*} |
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Wegen $\text{deg } (x^2-1)^{m} = 2^{m}$ folgt |
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Wegen $\text{deg } (x^2-1)^{m} = 2m$ folgt |
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$\frac{\mathrm{d}^{2m+1}}{\d x^{2m+1}}(x^2-1)^{m} = 0$. Damit folgt |
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\begin{align*} |
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\int_{-1}^{1} \frac{\mathrm{d}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}}{\d x^{m}}(x^2-1)^{m} \d x |
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\int_{-1}^{1} \frac{\mathrm{d}^{n}}{\d x^{n}}(x^2-1)^{n} |
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\frac{\mathrm{d}^{m}}{\d x^{m}}(x^2-1)^{m} \d x |
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&= 0 |
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.\end{align*} |
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\end{proof} |
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