# Approximation to a product of sequence $\prod_{i=0}^{n}(1-\sigma i)$ when $\sigma$ is very small

Is there a formula I can use as an approximation to the following equation for velocity of a projectile when $\sigma$ is very small?

$$\dot{x}[n] =v_0\prod_{i=0}^{n}(1-\sigma i)$$ $$\sigma\approx\frac{C_1}{1,000,000};C_1\in[0,100]$$

I am trying to compute the distance travelled (programmatically) when it is shot with initial velocity $v_0$.

The sum would be:

$$x[n]=\sum_{j=0}^{n}\dot{x}$$

or $$x[n]=v_0\sum_{j=0}^{n}\prod_{i=0}^{j}(1-\sigma i).$$

The solution for the sum according to a wolfram alpha computation is very complicated and computationally expensive. I derived this formula from the following function which updates the velocity every tick (10 milliseconds).

fire() {
frictiontimer = 1000000;
}

// This is called every 10ms
update() {
if (xspeed) {
int workfriction = frictiontimer / 1000;
xspeed = xspeed * workfriction / 1000;

frictiontimer -= FRICTION_CONSTANT;
if (frictiontimer < 0)
frictiontimer = 0;
}
}


Is there is a simplification I can use somewhere?

-
Can you please use brackets like: $\prod (1 - \sigma i)$ –  Aryabhata Apr 12 '12 at 19:48
When $\sigma i$ is small, $1 - \sigma i \approx \exp(-\sigma i)$, so $$\prod_{i=0}^n (1 - \sigma i) \approx \exp \left( - \sum_{i=0}^n \sigma i \right) = \exp(- \sigma n (n+1)/2)$$