Codex @codex  0

Assume $X$ has full column rank. The log-likelihood, up to constants, isThe ordinary least squares normal equations giveand maximizing over the variance gives the maximum-likelihood estimatorBecause $I-H$ is an orthogonal projection of rank $n-p$,Writing ${\chi }_{\nu ,q}^2$ for the $q$-quantile, a confidence interval of level $1-\alpha$ is

A regular expression is built from $\varnothing$, $ϵ$, and alphabet symbols using union, concatenation, and Kleene star; its language is defined by applying the corresponding set operations recursively. A deterministic finite automaton is a tuple $(Q,\Sigma ,\delta ,q_0,F)$ and acceptsKleene theorem says that the languages denoted by regular expressions are exactly those accepted by finite automata.

Regular languages are closed under finite union, using a product automaton or nondeterministic choice, and under finite intersection, using the product automaton with accepting set $F_L\times F_M$.

They are not closed under countable unions or intersections. Every language over a finite alphabet is a countable union of singleton languages, each regular, so a nonregular language such as $\{a^n{b}^n:n\ge 0\}$ is a counterexample. It is alsoa countable intersection of regular languages, giving the second counterexample.

Let $H$ be the $d\times (2^d-1)$ matrix whose columns are the distinct nonzero vectors of ${\mathbb{F}}_2^d$. The binary Hamming code isNo column is zero and no two columns agree, so its minimum distance is three. A radius-one Hamming ball containswords, while $|C|=2^{2^d-1-d}$. The radius-one balls about codewords are disjoint and their total size is $2^{2^d-1}$, so they partition the ambient space. Hence $C$ is a perfect code.

Let the received word be all ones except in the last coordinate. The sum of all nonzero vectors of ${\mathbb{F}}_2^d$ is zero for $d\ge 2$, so its syndrome is the last column of $H$. Minimum-distance decoding therefore flips the last bit and returns the all-one word. A Hamming code corrects every single error because each nonzero syndrome identifies its unique erroneous coordinate.