For any real number $\alpha\in\mathbb R$ and any two integers $x,y\in\mathbb Z$ with $y\neq 0,$ the following closed formula for the sum of floor function holds: $$\sum_{k=0}^{y-1}\left\lfloor\frac{xk+\alpha}m\right\rfloor=d\left\lfloor\frac{\alpha}d\right\rfloor +\frac{y-1}{2}x+\frac{d-y}{2},$$ where $d=\gcd(x,y)$ is the greatest common divisor of $x$ and $y.$
Proofs: 1
