I found a piece of code from a former colleague who seems to have copied it from another source (perhaps internet) that confused me a bit. Therein, the Certainty Equivalent (CE henceforth) for Cumulative Prospect Theory (here CPT) is calculated as
Function CEpos(cpt As Double, alpha As Double) As Double
If alpha > 0 Then
CEpos = cpt ^ (1 / alpha) 'OK
ElseIf alpha = 0 Then
CEpos = Exp(cpt)
Else
CEpos = ((1 - cpt) ^ (1 / alpha)) - 1
End If
End Function
Function CEneg(cpt As Double, alpha As Double, lambda As Double) As Double
If alpha > 0 Then
CEneg = (-1) * (((-1) * cpt / lambda) ^ (1 / alpha))
ElseIf alpha = 0 Then
CEneg = (-1) * Exp((-1) * cpt / lambda)
Else
CEneg = 1 - (1 + cpt / lambda) ^ (1 / alpha)
End If
End Function
I am confused as the CE of CPT is supposed to converge to $\lim_{\alpha\rightarrow{0}} (CE)=e^{CPT}$. Perhaps this is too simple but i'm stuck as my guess is that for ${\alpha\rightarrow{0}}$ the agent is insensitive to variations in gains/losses, thus the CE is zero.
My idea was that for $CPT={CE}^{\alpha}$ the CPT approaches 1 as i could expand the CPT using a Taylor Series and evalute at $\alpha=0$ such that \begin{equation} \label{eq:1} CPT=1+\alpha*\log(CE)+\frac{1}{2}*{\alpha^{2}}*\log^{2}(CE)+O(CE^{3}) \end{equation} and thus CPT is 1 for all gains (similar for losses, there shifted by the loss aversion parameter).
Would anyone mind helping me out how to get to the expression above from the code? I don't see it yet (and yes: it seems simple but as i said i'm stuck..)
Thanks a lot in advance
Thomas