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It turns out that all positive values for the time step are okay no matter how high.
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This is how similar to the backward Euler method
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and now we're going to look into why that is the case.
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The trapezoidal weir turns this differential equation into the following.
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The next value of x equals the current value of x plus the time step
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times the average of two rates of change, the current rate of change (-kx₁)
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and the next rate of change (-kx₂).
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I bring the x₂ to the left-hand side and combined this x₁ with that x₁
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and get (1+hk/2) * x2 = (1-hk/2) * x1.
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Let's check that, 1x₂, it's here. (hk/2)x₂ appears on the right-hand side with a - sign.
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1x₁ appears here minus (hk/2)x₁ - (hk/2)x₁ appears here.
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Correct. And that's easy to solve for x₂. x₂ = (1-hk/2)/(1+hk/2)*x₁.
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If you look at what happens with the next step. We of course, get a second factor of that sort.
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The next next step, another factor of that sort. So what happens is that we get powers of this factor.
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The question is what's going to happen about powers of that factor.
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So we see that to get it from x₁ to x₂, we have to multiple by this factor
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and this is going to occur every step if we want to go from x₂ to x₃,
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we again multiply by that factor and so on.
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So in the end, we get the powers of this factor.
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So the question is as the nth power of this expression converts to 0,
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as it intends to infinity to check that, one can analyze three different cases.
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Let's first check what happens if the step size is moderate.
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For instance, if this expression hk/2 equals 0.1 and we get (1-0.1)/(1+0.1)
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meaning (0.9)/(1.1), this is a positive number, <1.
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If you take higher and higher powers of that, this number is going to become 0.
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The numerator becomes 0.9. The denominator becomes 1.1.
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The fraction is a number between 0 and 1 and if we take higher and higher powers of such a number,
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the results converge to 0, so this is stable.
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So in this case, we have stability.
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If hk/2 happens to be equal to 1, we get a fraction with a 0 in the numerator,
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that's always 0, no problem with that, so we have stability in that case.
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And if you use a really high value of h, for instance, such that hk/2=9,
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we get something like 1-9=-8 divided by 1+9=10.
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This is a negative number with absolute value less than 1.
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So, if we take higher and higher powers, the result converges to 0,
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we also have stability in this case but you see that this convergence comes along with an oscillation
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that's time changes from minus to plus to minus to plus as we form even powers and odd powers.
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The result has a positive sign. For odd powers, the result has a negative sign.
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So we see some kind of decaying oscillation here.