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 Jul 4 comment The principle of duality for sets Thank you, this is exactly what I was looking for. I've been reading up on lattices now it is very interesting. Can you recommend any resources for learning more about lattice theory? Jul 4 accepted The principle of duality for sets Jul 1 asked The principle of duality for sets Jun 28 accepted Is every day tau day (or pi day) to some base? Jun 28 comment Is every day tau day (or pi day) to some base? @HagenvonEitzen oh. Duh. Jun 28 asked Is every day tau day (or pi day) to some base? Jun 20 accepted Find the Dirichlet inverse of the identity function Jun 18 asked Find the Dirichlet inverse of the identity function Jun 13 accepted Using Fermat's Little Theorem, find the least positive residue of $3^{999999999}\mod 7$ Jun 13 asked Using Fermat's Little Theorem, find the least positive residue of $3^{999999999}\mod 7$ May 28 revised If $x\equiv y \pmod{\gcd(a,b)}$, show that there is a unique $z\pmod{\text{lcm}(a,b)}$ with $z\equiv x\pmod a$ and $z\equiv y\pmod b$ edited title May 28 asked If $x\equiv y \pmod{\gcd(a,b)}$, show that there is a unique $z\pmod{\text{lcm}(a,b)}$ with $z\equiv x\pmod a$ and $z\equiv y\pmod b$ May 28 asked Prove that if $p_1,\dots,p_k$ are distinct odd primes then 1 has $2^k$ square roots $\mod m$ where $m$ is the product of the primes. May 23 comment Prove that if $n\geq\text{lcm}(a,b)$ and $\gcd(a,b)|n$ then $n=xa+yb$ for some integers $x,y\geq 0$ I'm still rather stuck. I can't quite see how I can show that the coefficients are nonnegative from that. Can you possibly offer any other insight? May 23 asked Prove that if $n\geq\text{lcm}(a,b)$ and $\gcd(a,b)|n$ then $n=xa+yb$ for some integers $x,y\geq 0$ May 17 awarded Nice Question Apr 22 accepted Is a broken clock right twice a day? Apr 22 comment Are the eigenvalues of $AB$ equal to the eigenvalues of $BA$? (Citation needed!) I know this is an ancient thread but hopefully you're still lurking out there somewhere. How come you need to hope that $\lambda\ne 0$? Doesn't the argument still work just fine in the case where $\lambda=0$? Apr 22 comment Is there a nice way to interpret this matrix equation that comes up in the context of least squares @Dolma thanks I fixed that Apr 22 revised Is there a nice way to interpret this matrix equation that comes up in the context of least squares deleted 8 characters in body