# Stochastic Differential Equation for Exponential Decay

#### Define a stochastic process satisfying the Ito stochastic differential equation . This models exponential decay subject to Wiener noise.

 In[1]:= X\[ScriptCapitalP] = ItoProcess[\[DifferentialD]x[ t] == -x[t] \[DifferentialD]t + \[Sigma] \[DifferentialD]w[t], x[t], {x, x0}, t, w \[Distributed] WienerProcess[]]
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#### Simulate the process for different values of the variance parameter .

 In[2]:= XTable[ListLinePlot[ RandomFunction[\[ScriptCapitalP] /. {x0 -> 5}, {0, 4, 0.01}, 10], PlotLabel -> Row[{"\[Sigma]", "\[Equal]", N@\[Sigma]}]], {\[Sigma], {1/4, 1/2, 1, 2}}]
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#### The mean function of the process is independent of .

 In[3]:= XMean[\[ScriptCapitalP][t]]
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#### This implies that the mean function coincides with the following deterministic solution.

 In[4]:= Xx[t] /. DSolve[{x'[t] == -x[t], x[0] == x0}, x[t], t]
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#### Find the variance function of the process .

 In[5]:= XVariance[\[ScriptCapitalP][t]]
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#### Find the probability density function of the value of the process .

 In[6]:= XPDF[\[ScriptCapitalP][t], x]
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