Frobenius number ($g(a_1,\dots ,a_k)$)

For positive integers $a_1,\dots ,a_k$ with greatest common divisor $1$, and with every $a_i>1$, the largest integer not expressible as a nonnegative integer linear combination of the $a_i$. For two coprime generators $a,b$, it equals $ab-a-b$.

For coprime integers $a,b>1$, the Frobenius number is $ab-a-b$. Every larger integer is a nonnegative integer linear combination of $a,b$.