[R] Fwd: Using odesolve to produce non-negative solutions

[Martin Henry H. Stevens]

Hi Spencer,
I have copied Woody Setzer. I have no idea whether lsoda can estimate  
parameters that could take imaginary values.
Hank
On Jun 11, 2007, at 12:52 PM, Spencer Graves wrote:

> <in line>
>
> Martin Henry H. Stevens wrote:
>> Hi Jeremy,
>> First, setting hmax to a small number could prevent a large step, if
>> you think that is a problem. Second, however, I don't see how you can
>> get a negative population size when using the log trick.
> SG:  Can lsoda estimate complex or imaginary parameters?
Hmm. I have no idea.
>
>> I would think that that would prevent completely any negative values
>> of N (i.e. e^-100000 > 0). Can you explain? or do you want to a void
>> that trick? The only other solver I know of is rk4 and it is not
>> recommended.
>> Hank
>> On Jun 11, 2007, at 11:46 AM, Jeremy Goldhaber-Fiebert wrote:
>>
>>> Hi Spencer,
>>>
>>> Thank you for your response. I also did not see anything on the  
>>> lsoda
>>> help page which is the reason that I wrote to the list.
>>>
>>>> From your response, I am not sure if I asked my question clearly.
>>>
>>> I am modeling a group of people (in a variety of health states)
>>> moving through time (and getting infected with an infectious
>>> disease). This means that the count of the number of people in each
>>> state should be positive at all times.
>>>
>>> What appears to happen is that lsoda asks for a derivative at a  
>>> given
>>> point in time t and then adjusts the state of the population.
>>> However, perhaps due to numerical instability, it occasionally lower
>>> the population count below 0 for one of the health states (perhaps
>>> because it's step size is too big or something).
>>>
>>> I have tried both the logarithm trick
> <snip>
>
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Dr. Hank Stevens, Assistant Professor
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