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math121b:02-19 [2020/02/19 01:10] pzhou created |
math121b:02-19 [2020/02/19 01:21] (current) pzhou |
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**Naive Approach** | **Naive Approach** | ||
- | If for some $\lambda$, we can solve to get $A_{\lambda, | + | If for some $\lambda$, we can solve to get $A_{\lambda, |
$$F_{\lambda, | $$F_{\lambda, | ||
and we may hope to express any solution as | and we may hope to express any solution as | ||
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$$ C_n = \frac{2}{\sinh(n \pi)} \int_0^1 f(x) \sin(n \pi x) dx. $$ | $$ C_n = \frac{2}{\sinh(n \pi)} \int_0^1 f(x) \sin(n \pi x) dx. $$ | ||
- | I feel a bit lucky that this can be solved. | + | ** General Boundary Value? **: suppose the four sides of the square has non-zero values? Just break the problem to 4 smaller problems, where each one has only one side non-zero, then do this problem 4 times. |
+ | |||
+ | ===== Eigenvalue problem ===== | ||
+ | The equation | ||
+ | $$ \d_x^2 X(x) = \lambda X(x), \quad X(0) = 1, \quad X(1)=0 $$ | ||
+ | is an eigenvalue problem. For a randomly chosen $\lambda$, say $0.721$, the solution is only $0$. Hence to have a non-zero solution, $\lambda$ | ||
+ | |||
+ | This is analogous to the problem of solving for eigenvalue and eigenvector for an $n \times n$ matrix $A$: | ||
+ | $$ A v = \lambda v$$ | ||
+ | here, recall we solve for the roots of the $P(\lambda) = \det(A - \lambda I) = 0$, then for each $\lambda$, we solve for $v$. (There might be several linearly independent $v$ corresponding to the same $\lambda$). | ||
+ | |||
+ | Here we have the same problem, but, instead of having a matrix $A$ acting on a vector space $\R^n$, we have | ||
+ | an operator $\d_x^2$ acting on (an infinite dimensional vector space) the function space on $[0,1]$. | ||
+ | |||