# White noise for 'SDEs with diagonal noise'

dear all,
I have a SDEs with a stochastic part as a white noise k*N(0,1).
Time step is ‘dt’
Following explanations here I don’t get why the white noise is N(0,dt) and not N(0,1/dt), which seems consitent with my results as when using in the sde code k*dt my outputs make more sense than using k*1/dt … where am I wrong ??
Thx !
Frédéric

What do you mean? It’s just using the standard definition.

I am trying to represent seismic excitation as input for my SDEs.
To simplify, this excitation can be represented as a white noise with an intensity of So (m2/s3), the only parameter I know
For a given time step, the SDE concerned with this white noise is something like
Mx.. + Cx. + Kx = sqrt(So/dt)*N(0,1)
Right part is then, according to explanations for SDEs with Diagonal Noise something like
du[1] = sqrt(S0/dt)*N(0,dt)/dt
results are then unconsitent with experiments, while
du[1] = sqrt(S0/dt)*N(0,dt)*dt
are !
Is there something I have missed ??
Thx for the help !
Frédéric

This doesn’t make sense. `Mx.. + Cx. + Kx = sqrt(So/dt)*N(0,1)` that’s not a well-defined equation. What is the behavior as `dt->0`? That diverges to infinite variance in infinitesimal steps!

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dt is fixed in my case, (exactly 10^-2s)

For starters, is this the variance or the standard deviation of the white noise in a step?

it’s the constant part (intensity) of the spectral density function representing the seismic excitation
for exemple described in this paper
(I am trying the easiest model, not yet Clough-Penzien )

… so the variance inside a step

But continuous equations don’t have steps, so you can’t define the equation by what happens in a “step”. This is wild.

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yes of course, I was just writing the equation that way for someone to understand the sqrt(So/dt) part as it is the homogenous with an acceleration
the continuous equation in that case is
Mx.. + Cx. + Kx = W, with W Gaussian White Noise Process

Then W ~ N(0,dt) or sqrt(dt)*N(0,1). It seems like what you’re really saying is that your equation is:

``````Mx.. + Cx. + Kx = sqrt(S0/1e-4)*W
``````

?

ok I get it now
as W~N(0,dt) → sqrt(dt)*N(0,1) it is clear now why
sqrt(S0/dt)*W → sqrt(S0/dt)*sqrt(dt)*N(0,1) is working well, and NOT sqrt(S0/dt)*N(0,1)/sqrt(dt)
very sorry for this basic question and thx a lot for the help !