I get the follow equation in a paper. Let $A \in \mathbb{R}^{2 \times 2}$, then $M = A^TA$ is a positive semi-definite matrix, the nuclear norm of $A$ is:


$$ \Vert A \Vert\_\* = \sqrt{\operatorname{tr}(M) + 2\sqrt{\det(M)}}.$$


Are there any analytical form of nuclear norm for $n \times n$ matrix like the above form? Or just for a $3 \times 3$ matrix?



Analytical form for the nuclear norm of an $n \times n$ matrix
**As Gro-Tsen pointed out, I had not computed the Galois group of the governing polynomial, and, in fact, my original answer was wrong. I believe that the following answer is correct, though.**


If by 'like the above form' you mean an expression in terms of radicals of the traces of the powers of $M$, the answer is 'no' for $n>5$. Even for $n=3$ and $n=4$, when there is such an expression, it is very ugly and will almost certainly be useless for any practical purpose:


Let $p\_i = \mathrm{tr}(M^i)$ for $1\le i\le n$, and let $q = \|A\|\_\*$. If $\lambda\_1,\ldots,\lambda\_n\ge 0$ are the eigenvalues of $M$, then $p\_i = {\lambda\_1}^i+\cdots+{\lambda\_n}^i$ and $q = \sqrt{\lambda\_1}+\cdots+\sqrt{\lambda\_n}$. The problem is to compute the Galois group of the minimal polynomial of $q$ in the field generated by the $p\_i$ and determine whether it is solvable, since this will determine whether $q$ can be expressed algebraically in terms of the $p\_i$ using only radicals.


To clarify things, write $\mu\_i = \sqrt{\lambda\_i}$, so that $q = \mu\_1 + \cdots + \mu\_n$ while $p\_i = {\mu\_1}^{2i}+\cdots+{\mu\_n}^{2i}$.


Let $\mathbb{S}\_n\subset\mathrm{O}(n)$ be the signed permutation group acting linearly on the span of the $\mu\_i$. Thus, $\sigma\in \mathbb{S}\_n$ satisfies $\sigma(\mu\_i) = \epsilon\_i\,\mu\_{\pi(i)}$ where $\epsilon\_i=\pm1$ and $\pi$ is a permutation of $\{1,\ldots,n\}$. Then $\sigma$ extends to an automorphism of the field $\mathbb{Q}(\mu\_1,\ldots,\mu\_n)$ whose fixed field is easily seen to be $\mathbb{Q}(p\_1,\ldots,p\_n)$.
Let $(\mathbb{Z}\_2)^n\subset\mathbb{S}\_n$ be the abelian normal subgroup consisting of the $\sigma$ of the form $\sigma(\mu\_i) = \epsilon\_i\,\mu\_{i}$ where $\epsilon\_i=\pm1$.


Consider the polynomial
$$
f(t) = \prod\_{\sigma\in(\mathbb{Z}\_2)^n}\bigl(t-\sigma(q)\bigr).
$$
Then $f(q) = 0$. The $t$-degree of $f$ is $2^n$, and it is easy to see that the $t$-coefficients of $f$ lie in $\mathbb{Q}(p\_1,\ldots,p\_n)$ (since they are symmetric polynomials in ${\mu\_1}^2,\ldots,{\mu\_n}^2$). Moreover, the set $R=\{\sigma(q)\ |\ \sigma\in (\mathbb{Z}\_2)^n\}$ consists of $2^n$ distinct elements, which are the roots of $f$ in $\mathbb{Q}(\mu\_1,\ldots,\mu\_n)$. The group $\mathbb{S}\_n$ acts transitively on $R$, and the $\mathbb{Q}$-linear span of $R$ contains $\mu\_1,\ldots,\mu\_n$, so $\mathbb{Q}(\mu\_1,\ldots,\mu\_n)$ is the splitting field of $f$. (Hence $f$ must be irreducible in $\mathbb{Q}(p\_1,\ldots,p\_n)[t]$.)


If $\alpha$ is an automorphism of $\mathbb{Q}(\mu\_1,\ldots,\mu\_n)$ that preserves the set $R$, then it must preserve its $\mathrm{Q}$-linear span and it must preserve the sum of the squares of the elements of $R$, which is clearly $2^n({\mu\_1}^2 + \cdots + {\mu\_n}^2)$. From this, it is easy to see that $\alpha$ must preserve the set $\{\pm\mu\_1,\ldots,\pm\mu\_n\}$ and hence must belong to $\mathbb{S}\_n$. Thus, the Galois group of $f$ is $\mathbb{S}\_n$.


Since $\mathbb{S}\_n/(\mathbb{Z}\_2)^n$ is isomorphic to the permutation group $S\_n$ and since $(\mathbb{Z}\_2)^n$ is abelian, it follows that $\mathbb{S}\_n$ is solvable if and only if $S\_n$ is solvable. Of course, $S\_n$ is solvable if and only if $n<5$.


