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Considered a categorical distribution, which is a discrete distribution over $K$ outcomes (i.e. $1$ through $K$). The probability of each category is explicitly represented with parameter $\theta_k$. Which we have that $\theta_k \geq 0$ and $\sum_{k=1}^{K} \theta_k = 1$.
Also each observation $\textbf{x}$ is a $1$-of-$K$ encoding, (i.e $\textbf{x}$ a vector where one of the entries is $1$ and the rest are $0$). Under this model, the probability of an observation can be written in the following form: $p(\textbf{x}|\theta)=\prod_{k=1}^{K}\theta_k^{x_k}$.
Assume that the dataset is $\mathcal{D}=\{ \textbf{x}^{(1)},...,\textbf{x}^{(N)} \}$.
Which we denote that the count for each outcome $k$ as $N_k = \sum_{i=1}^N x_k^{(i)}$ and $N = \sum_{k=1}^K N_k$.
Note that each data point is in the $1$-of-$K$ encoding,
(i.e. $x_k^{(i)} = 1$ if the $i$-th datapoint represents an outcome $k$, otherwise $x_k^{(i)} = 0$).
Assume that $N_k > 0$, Find the maximum likelihood estimator (MLE) $\hat{\theta}_k$.

$\textbf{My Attempt:}$
Since, notice that varibales in $\mathcal{D}$ are independent and identically distributed notice that likelihood function $L(\theta) = p(\text{x}^{(1)},...,\text{x}^{(N)}~|~\theta_1,...,\theta_K) = p(\text{x}^{(1)},...,\text{x}^{(N)}, \theta_1,...,\theta_K)$.
So, the log-likelihood is $l(\theta) = \log\big(p(\text{x}^{(1)},...,\text{x}^{(N)}, \theta_1,...,\theta_K)\big)$.
Since $\sum_{k=1}^{K} \theta_k = 1$. Then, $\theta_K = 1 - \sum_{k=1}^{K-1} \theta_k$.
Then, $$l(\theta) = \sum_{i=1}^{N}\bigg[x_1^{(i)}\log(\theta_1) + \cdots + x_{K-1}^{(i)}\log(\theta_{K-1}) + x_{K}^{(i)}\log\bigg(1-\sum_{k=1}^{K-1}\theta_k\bigg)\bigg]$$
Since, $N_k = \sum_{i=1}^N x_k^{(i)}$ and $N = \sum_{k=1}^K N_k$.
Then, \begin{align*} l(\theta) &= \sum_{i=1}^{N}\bigg[x_1^{(i)}\log(\theta_1) + \cdots + x_{K-1}^{(i)}\log(\theta_{K-1})\bigg] + \sum_{i=1}^{N}\bigg[x_{K}^{(i)}\log\bigg(1-\sum_{k=1}^{K-1}\theta_k\bigg)\bigg] \\ &= \bigg[N_1\log(\theta_1) + \cdots + N_{K-1}\log(\theta_{K-1})\bigg] + N_K\log\bigg(1-\sum_{k=1}^{K-1}\theta_k\bigg) \end{align*} Then, by Lagrange multiplier,
we can set $f(\theta_1,...,\theta_{K-1})=N_1\log(\theta_1) + \cdots + N_{K-1}\log(\theta_{K-1})$ and $g(\theta_1,...,\theta_{K-1}) = N_K\log\bigg(1-\sum_{k=1}^{K-1}\theta_k\bigg)$.
Also, $\lambda = 1$ which that $l(\theta)=f(\theta_1,...,\theta_{K-1})+\lambda g(\theta_1,...,\theta_{K-1})$.
So, the $\nabla l(\theta) = \big(\frac{\partial l}{\partial \theta_1},...,\frac{\partial l}{\partial \theta_{K-1}}, \frac{\partial l}{\partial \lambda}\big)$.
Which we have $\nabla l(\theta) = \bigg(\frac{N_1}{\theta_1} - \frac{N_K}{1-\sum_{k=1}^{K-1}\theta_k},..., \frac{N_{K-1}}{\theta_{K-1}} - \frac{N_K}{1-\sum_{k=1}^{K-1}\theta_k}, N_K\log\big(1-\sum_{k=1}^{K-1}\theta_k\big)\bigg)$.
Since, $\theta_K = 1 - \sum_{k=1}^{K-1} \theta_k$.
So, $\nabla l(\theta) = \bigg(\frac{N_1}{\theta_1} - \frac{N_K}{\theta_K},..., \frac{N_{K-1}}{\theta_{K-1}} - \frac{N_K}{\theta_K}, N_K\log\big(1-\sum_{k=1}^{K-1}\theta_k\big)\bigg)$.
Then, setting all the terms to $0$.
We have that \begin{align*} \frac{N_1}{\theta_1} - \frac{N_K}{\theta_K} = 0 &\implies N_1\theta_K - N_K\theta_1 = 0 \\ &~~~~~~~\vdots \\ \frac{N_{K-1}}{\theta_{K-1}} - \frac{N_K}{\theta_K} = 0 &\implies N_{K-1}\theta_K - N_K\theta_{K-1} = 0 \end{align*} Since, we assumed that $N_k > 0$. So, $N_K\log\big(1-\sum_{k=1}^{K-1}\theta_k\big) = 0$ $\implies \log\big(1-\sum_{k=1}^{K-1}\theta_k\big) = 0 \implies 1-\sum_{k=1}^{K-1}\theta_k = 1 \implies \theta_K = 1$.
So, sub the $\theta_K = 1$ to above equations, we have the maximum likelihood estimators
$\hat{\theta}_1=\frac{N_1}{N_K}$, $\cdots$, $\hat{\theta}_{K-1}=\frac{N_{K-1}}{N_K}$ and $\hat{\theta}_K = 1$.

But I don't feel this is correct, since I think the correct answer should be $\hat{\theta}_k=\frac{N_k}{N_1+\cdots+N_K}$ for each $k \in \{1,...,K\}$.
So, where did I do wrong and what should I fix to be right ?

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You are using Lagrangian multipliers incorrectly. For this model I recommend not using Lagrangian multipliers but simply the reparametrization of $K-1$ parameters $\boldsymbol{\theta} = \left(\theta_1, \cdots, \theta_{K-1}\right)$. Any inference for $\theta_K$ can be obtained by the invariance property of the MLEs since it is a function of these $K-1$ parameters.

So your gradient is correct, ignoring the last term; however, you must replace $\theta_K$ with $1 - \sum_{i=1}^{K-1} \theta_i$. Now you will have a linear system of $K-1$ equations and parameters, which can be solved. Let $\boldsymbol{1}_{K-1} \in \mathbb{R}^{K-1}$ denote a vector of 1's. You should arrive at a formula that looks like \begin{eqnarray*} \left(N_K Diag\left(\frac{1}{N_1}, \cdots, \frac{1}{N_{K-1}}\right) +\boldsymbol{1}_{K-1}\boldsymbol{1}_{K-1}^{\prime}\right)\widehat{\boldsymbol{\theta}} &=& \boldsymbol{1}_{K-1} \\ \widehat{\boldsymbol{\theta}} &=& \begin{pmatrix} \frac{N_1}{N_k} \\ \vdots \\ \frac{N_{K-1}}{N_K} \end{pmatrix}, \end{eqnarray*} where the second equality follows from the Sherman-Morrison formula and matrix algebra.

If you want to use Lagrangian multipliers then one does not need to change the parameter space. Let $\boldsymbol{\theta}^{\ast} = \left(\theta_1, \cdots, \theta_K \right)$. We wish to optimize the likelihood in the reduced domain such that $\sum_{i=1}^K \theta_i = 1$. One way to code the Lagrange multiplier and restraint function is to subtract the term $\lambda(\sum_{i=1}^K \theta_i -1)$ from the log-likelihood. Therefore, the log-likelihood with the Lagrange multiplier is \begin{eqnarray*} l(\boldsymbol{\theta}^{\ast}, \lambda) &=& \sum_{i=1}^K N_i \log \left(\theta_i\right) - \lambda(\sum_{i=1}^K \theta_i -1) \end{eqnarray*} Of interest are the following partial derivatives: \begin{eqnarray*} \frac{\partial l(\boldsymbol{\theta}^{\ast}, \lambda)}{\partial \theta_j} &=& \frac{N_i}{\theta_i} - \lambda \quad \mbox{for} \quad j=1,\cdots,K\\ \frac{\partial l(\boldsymbol{\theta}^{\ast}, \lambda)}{\partial \lambda} &=& 1- \sum_{i=1}^K \theta_i \end{eqnarray*} Hence summing over all $K$ partial derivatives in the first equation, after equating them to 0, one obtains \begin{eqnarray*} \lambda \sum_{i=1}^K \theta_i &=& N\\ \lambda &=& N, \end{eqnarray*} where the second equation holds by setting $\frac{\partial l(\boldsymbol{\theta}^{\ast}, \lambda)}{\partial \lambda}$ to 0. Now plug-in $\lambda$ back to the first partial derivative equations to obtain the MLEs.

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