# Finding Expectation of function of a Log-normal distribution

Say $Y=g(X)$ and $p_X = \frac{e^{-\frac{(\mu -\log (x))^2}{2 \sigma ^2}}}{\sqrt{2 \pi } x \sigma }$ is Log-normal density function: [Wiki]

Find E[Y]?

Since $E[Y] = \int_0^\infty y f_Y \ dy = \int_0^\infty g(x)f_X(x)dx$. So, I do not need to find $f_Y(y)$.

.
px = Simplify[PDF[LogNormalDistribution[Mu, Sigma], x], x > 0]
Integrate[Log2[1 + x] px, {x, 0, \Infinity}, Assumptions -> {Sigma > 0}]
.


Unfortunately, Mathematica cannot evaluate this integral for a closed form expression and I'm not quite sure how else to solve it. Will appreaciate any help I can get on the evaluation steps.

Edit:

 Expectation[Log2[1 + x], x \[Distributed] LogNormalDistribution[Mu, Sigma]]


I suspect there may not be a closed form expression (I have not looked at it hard enough). If the aim is to not analytical but numerical, I post the following for illustration (apologies if not the intent of question):

f[x_, y_] :=
NIntegrate[
Log2[t + 1] Exp[-(x - Log[t])^2/(2 y^2)]/(Sqrt[2 Pi] t y), {t, 0,
Infinity}]
rv[a_, b_, n_] :=
Mean[Log2[1 + RandomVariate[LogNormalDistribution[a, b], n]]]
Manipulate[
Show[Plot[f[m, s], {m, 1, 5}, PlotRange -> {0, 7}],
ListPlot[Table[{j, rv[j, s, n]}, {j, 1, 5, 0.1}],
PlotMarkers -> {"\[FilledDiamond]", 10}, PlotStyle -> Red]], {s, 1,
5}, {n, {10, 100, 1000, 10000}}] You can also depict the expected value nicely as a function of Mu and Sigma using Plot3D.

px = Simplify[PDF[LogNormalDistribution[Mu, Sigma], x], x > 0]

E^(-((Mu - Log[x])^2/(2 Sigma^2)))/(Sqrt[2 π] Sigma x)

Plot3D[NIntegrate[Log2[1 + x] px, {x, 0, ∞}], {Mu, -5,
10}, {Sigma, .2, 2}, AxesLabel -> {Mu, Sigma, EY}] PS: Reminds me of Black-Scholes formula for the evaluation of an Option.

Extending the range of Sigma Shows the nontrivial dependence more clearly: 