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**pococz****Member**- Registered: 2010-07-18
- Posts: 8

hello.. i'm just confused about how to transform continuous form(differential equation) to discrete form

ex :

dS/dt = (alpha)*n- (alpha)*S - (beta)*S*I

dI/dt = (beta)*S*I- (gamma)*I

dR/dt = (gamma)*I

become

S = ??

I = ??

R =??

thank you very much...

*Last edited by pococz (2010-07-18 14:32:40)*

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 87,194

Hi pococz;

I am a little confused here. As far as I know you can convert a DE ( continuous ) into a Recurrence ( discrete ), you don't convert the constants. I don't understand what you are trying to accomplish. Can you give more? What textbook or webpage are you studying from?

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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**pococz****Member**- Registered: 2010-07-18
- Posts: 8

i'm sorry.. i've just read epidemic modelling books by daley and gani... there are some discrete model on that book...

my exercise is to analyze the fixed point stability from discrete time epidemic model.but the model on that book can't be anlyzed, so i think to change from the continuous model to discrete model.. i'm so thankful..

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 87,194

Hi pococz;

I have that book, please tell me where in it are you exactly working! Maybe I can help out.

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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**pococz****Member**- Registered: 2010-07-18
- Posts: 8

ooohh.. you have that book. please open the 50th page.. there is a deterministic model in discrete time for epidemics problem. can you show me how to find the stability of the model. especially if you can solve the stability depends on the Ro (reproduction number)...

so please.. help me dude..

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 87,194

Hi pococz;

I have the 1999 edition. My pahe 50 has a figure 2.12 on the top. Am I in the right spot?

**In mathematics, you don't understand things. You just get used to them.Of course that result can be rigorously obtained, but who cares?Combinatorics is Algebra and Algebra is Combinatorics.**

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**pococz****Member**- Registered: 2010-07-18
- Posts: 8

yes.. you are on the spot... thank's

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 87,194

Hi;

I am already stuck on how he derived:

Of course that result can be rigorously obtained, but who cares?

Combinatorics is Algebra and Algebra is Combinatorics.

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**pococz****Member**- Registered: 2010-07-18
- Posts: 8

yup.. but from the first model at above (continuous model ) can you change it to discrete form (like in 50th page)..

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 87,194

Hi;

Do you have some initial conditions for post#1. If I have initial conditions I might be able to.

Of course that result can be rigorously obtained, but who cares?

Combinatorics is Algebra and Algebra is Combinatorics.

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**pococz****Member**- Registered: 2010-07-18
- Posts: 8

hmmm.... how about semi discretization??

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 87,194

Hi pococz;

Okay you got me, what is a semi discretization?

Of course that result can be rigorously obtained, but who cares?

Combinatorics is Algebra and Algebra is Combinatorics.

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