Order in a number field
= Order in a number field
{title2=$[\mathcal O_K:A]<\infty$}
An order in a <number field> $K$ is a subring $A\subseteq\mathcal O_K$ containing $1$ and having full rank $[K:\mathbb Q]$ as an additive <abelian group>. Its finite index in the <ring of integers of a number field> is related to its <discriminant> by the <discriminant-index formula for an integral lattice>.