The [Hadamard circulant conjecture](https://mathoverflow.net/questions/55524/status-of-the-hadamard-circulant-conjecture) states that there do not exist circulant Hadamard matrices with more than $4$ columns. 


An $n$ by $n$ Hadamard matrix where the entries are chosen from $\{-1,1\}$ has determinant $\pm n^{n/2}$.



> 
> Does there exist a $c >1$ for which there exists an infinite number of positive integers $n$ such that
>  there exists an $n$ by $n$ circulant $\{-1,1\}$-matrix with
>  determinant at least $n^{n/c}$ (in absolute value)?
> 
> 
> 


Despite searching the literature as far as possible I have not found this question discussed. What is its status? In particular, is it the case that:


* It is known to be true.
* It is conjectured to be true.
* It is conjectured to be not true.
* No one has ever considered the question.
* Other.


Thank you very much for any help.



Bounds for maximum determinant of circulant matrices
I have not seen this question discussed in the literature either, but I will propose an answer here.


To begin with, I think that since $n^{n/2}$ is the maximum possible absolute value that the determinant of *any* $\{-1,1\}$-matrix can have (Hadamard upper bound), the constant $c$ in the question (if it exists) must necessarily be $\geq 2$. 


I will now argue that *any* $c>2$ has the property described in the question. I will do that by sketching a derivation of the following lower bound for the largest $|\det A|$, which is stronger asymptotically for any $c>2$ than the bound $n^{n/c}$ suggested in the question. 


**Proposition:**



> 
> There exists a real constant $\alpha > 0$, so that for any positive integer $n \geq 3$ there is at least one circulant matrix $A \in \{-1,1\}^{n \times n}$ that obeys $|\det A| > (\alpha n)^{n/2}$. Moreover, $\alpha = \sqrt2/{\rm e} \approx 0.52$ (and thus also any smaller $\alpha$) has this property.
> 
> 
> 


The above proposition can be proved as follows. Using the same technique as described in [this previous answer](https://mathoverflow.net/questions/210712/expected-size-of-determinant-of-aat-for-random-circulant-and-toeplitz-matrice/228221#228221 "Answer to: 'Expected size of determinant of AA^T for random circulant and Toeplitz matrices'"), it can be shown that the mean value of $|\det A|^2$ over all $n \times n$ circulant $\{-1,1\}$-matrices $A$ is bounded from below by $2^{\lfloor \frac{n-1}2 \rfloor} n!$, for any $n$ except $n=2$, where that mean value is zero. (In the [previous answer](https://mathoverflow.net/questions/210712/expected-size-of-determinant-of-aat-for-random-circulant-and-toeplitz-matrice/228221#228221 "Answer to: 'Expected size of determinant of AA^T for random circulant and Toeplitz matrices'"), a lower bound $2^{-2n+\lfloor \frac{n-1}2 \rfloor} (n+1)!$ was derived for circulant $\{0,1\}$-matrices instead.) 


Using Stirling's lower bound on $n!$ we then obtain the following sequence of lower bounds on the mean value of $|\det A|^2$:
$$
2^{\lfloor \frac{n-1}2 \rfloor} n! > 2^{\frac{n}2-1} \sqrt{2\pi n}\ \left(\frac{n}{\rm e}\right)^n > \left(\frac{\sqrt2}{\rm e} n \right)^n \ . 
$$
The last of these bounds implies that among these matrices $A$ there must be at least one with $|\det A| > (\alpha n)^{n/2}$, where $\alpha =\sqrt2/{\rm e}$, which concludes the proof. 


