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4 | 4 | % The CUSP problem :cite:p:`HW96` (pp. 147-148) is the PDE |
5 | 5 | % |
6 | 6 | % $$ |
7 | | - % \frac{\partial y}{\partial t} &= -\frac{1}{\varepsilon} (y^3 + a y + b) |
8 | | - % + \sigma \frac{\partial^2 y}{\partial x^2}, \\ |
9 | | - % \frac{\partial a}{\partial t} &= b + 0.07 v + \sigma \frac{\partial^2 a}{\partial x^2}, \\ |
10 | | - % \frac{\partial b}{\partial t} &= (1 - a^2) b - a - 0.4 y + 0.035 v + \sigma \frac{\partial^2 b}{\partial x^2}, |
| 7 | + % \frac{\partial y}{\partial t} &= -\frac{1}{ε} (y^3 + a y + b) + σ \frac{\partial^2 y}{\partial x^2}, \\ |
| 8 | + % \frac{\partial a}{\partial t} &= b + 0.07 v + σ \frac{\partial^2 a}{\partial x^2}, \\ |
| 9 | + % \frac{\partial b}{\partial t} &= (1 - a^2) b - a - 0.4 y + 0.035 v + σ \frac{\partial^2 b}{\partial x^2}, |
11 | 10 | % $$ |
12 | 11 | % |
13 | 12 | % |
14 | 13 | % where $v = u / (u + 0.1)$ and $u = (y - 0.7)(y - 1.3)$. The spatial domain $x \in [0, 1]$ has period boundary |
15 | 14 | % conditions. Discretization with second order finite difference on a grid with $N$ cells gives |
16 | 15 | % |
17 | 16 | % $$ |
18 | | - % y'_i &= -\frac{1}{\varepsilon} (y_i^3 + a_i y_i + b_i) + \sigma N^2 (y_{i-1} - 2 y_i + y_{i+1}) \\ |
19 | | - % a'_i &= b_i + 0.07 v_i + \sigma N^2 (a_{i-1} - 2 a_i + a_{i+1}) \\ |
20 | | - % b'_i &= (1 - a_i^2) b_i - a_i - 0.4 y_i + 0.035 v_i + \sigma N^2 (b_{i-1} - 2 b_i + b_{i+1}), |
| 17 | + % y'_i &= -\frac{1}{ε} (y_i^3 + a_i y_i + b_i) + σ N^2 (y_{i-1} - 2 y_i + y_{i+1}) \\ |
| 18 | + % a'_i &= b_i + 0.07 v_i + σ N^2 (a_{i-1} - 2 a_i + a_{i+1}) \\ |
| 19 | + % b'_i &= (1 - a_i^2) b_i - a_i - 0.4 y_i + 0.035 v_i + σ N^2 (b_{i-1} - 2 b_i + b_{i+1}), |
21 | 20 | % $$ |
22 | 21 | % |
23 | 22 | % where $i = 1, \dots, N$. Values at cell indices $i=0, N+1$ are specificied by the periodic boundary conditions. |
24 | 23 | % |
25 | 24 | % Notes |
26 | 25 | % ----- |
27 | | - % +---------------------+----------------------------------------------------------------------------+ |
28 | | - % | Type | PDE | |
29 | | - % +---------------------+----------------------------------------------------------------------------+ |
30 | | - % | Number of Variables | arbitrary multiple of 3 | |
31 | | - % +---------------------+----------------------------------------------------------------------------+ |
32 | | - % | Stiff | typically, depending on $\varepsilon$, $\sigma$, and number of grid points | |
33 | | - % +---------------------+----------------------------------------------------------------------------+ |
| 26 | + % +---------------------+-------------------------------------------------------------+ |
| 27 | + % | Type | PDE | |
| 28 | + % +---------------------+-------------------------------------------------------------+ |
| 29 | + % | Number of Variables | arbitrary multiple of 3 | |
| 30 | + % +---------------------+-------------------------------------------------------------+ |
| 31 | + % | Stiff | typically, depending on $ε$, $σ$, and number of grid points | |
| 32 | + % +---------------------+-------------------------------------------------------------+ |
34 | 33 | % |
35 | 34 | % Example |
36 | 35 | % ------- |
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44 | 43 | end |
45 | 44 |
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46 | 45 | properties (SetAccess = private) |
47 | | - % Right-hand side containing the diffusion terms and the reaction terms multiplied by $\varepsilon^{-1}$. |
| 46 | + % Right-hand side containing the diffusion terms and the reaction terms multiplied by $ε^{-1}$. |
48 | 47 | % |
49 | 48 | % This partition of the RHS is used in :cite:p:`JM17`. |
50 | 49 | % |
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53 | 52 | % RHSNonstiff |
54 | 53 | RHSStiff |
55 | 54 |
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56 | | - % Right-hand side containing the reaction terms not scaled by $\varepsilon^{-1}$. |
| 55 | + % Right-hand side containing the reaction terms not scaled by $ε^{-1}$. |
57 | 56 | % |
58 | 57 | % This partition of the RHS is used in :cite:p:`JM17`. |
59 | 58 | % |
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