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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

8 votes

Are these two methods for constructing Hadamard matrices known?

Two matrices are Hadamard equivalent if they differ from one another by permuting and negating rows and columns. It is known that there is a unique Hadamard equivalence class of Hadamard matrices at o …
Padraig Ó Catháin's user avatar
3 votes

A family of difference sets (paper by A. L. Whiteman)

The multiplicative group of $\mathbb{Z}_{pq} \cong \mathbb{Z}_{p} \times \mathbb{Z}_{q}$ is not cyclic, but is generated by $x =(a,1)$ and $y = (1,b)$ where $a$ is a primitive root mod $p$ and $b$ is …
Padraig Ó Catháin's user avatar
2 votes

Permutations of $(Z/pZ)^*$

A similar concept is an orthomorphism of a group $G$. This is an automorphism $\theta: G \rightarrow G$ with the property that $g^{-1}\theta(g)$ is a bijection (equivalently an automorphism). Two orth …
Padraig Ó Catháin's user avatar