what if f(y) = exp(y(theta) - e^(theta) - ln(y!)), find E[ln(y)] in terms of theta

We are given the probability mass function (PMF) of a discrete random variable \( Y \) as:

\[
f(y) = \exp\left( y \theta - e^{\theta} - \ln(y!) \right)
\]

This resembles the form of the probability mass function for a **Poisson distribution** with parameter \( \lambda = e^{\theta} \). Let's verify this by comparing with the Poisson PMF:

\[
P(Y = y) = \frac{e^{-\lambda} \lambda^y}{y!}
\]

Substitute \( \lambda = e^{\theta} \):

\[
P(Y = y) = \frac{e^{-e^{\theta}} (e^{\theta})^y}{y!} = \exp\left( y \theta - e^{\theta} - \ln(y!) \right)
\]

Thus, \( Y \) follows a Poisson distribution with mean \( \lambda = e^{\theta} \).

### Step 1: Find \( E[\ln(Y)] \)

For a Poisson-distributed random variable \( Y \sim \text{Poisson}(\lambda) \), where \( \lambda = e^{\theta} \), we need to compute the expected value \( E[\ln(Y)] \).

There is no simple closed form for \( E[\ln(Y)] \) for a Poisson distribution, but it is known that:

\[
E[\ln(Y)] = \psi(\lambda)
\]

where \( \psi(\lambda) \) is the **digamma function**, and \( \lambda = e^{\theta} \). Therefore:

\[
E[\ln(Y)] = \psi(e^{\theta})
\]

### Final Result:
\[
E[\ln(Y)] = \psi(e^{\theta})
\]

where \( \psi(x) \) is the digamma function evaluated at \( e^{\theta} \).

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Would you like to explore any particular details or further clarifications? Here are a few related questions to expand on this:

1. What is the digamma function \( \psi(x) \) and how is it defined?
2. How can we approximate \( E[\ln(Y)] \) for large values of \( \theta \)?
3. Can we find \( E[Y] \) and \( \text{Var}(Y) \) for this Poisson distribution?
4. How is the digamma function related to the gamma function?
5. Could you explain how the moment generating function (MGF) for the Poisson distribution relates to this?

**Tip**: The digamma function \( \psi(x) \) is the derivative of the logarithm of the gamma function and plays an important role in various statistical calculations.

Explore how to derive the expected value of the logarithm of a Poisson-distributed random variable. This problem involves advanced probability theory concepts, such as the Poisson distribution and digamma function, making it suitable for advanced undergraduate or graduate students.

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