A recent trend in advertising is viral marketing. The goal is to convince viewers to share an amusing advertisement by e-mail or
social networking. Imagine that the video is sent to 130 people on day 1. Each person agrees to send the video to 4 people the next day, and to request that each of those people send it to 4 people the day after they receive it. The number of viewers V(n) is an exponential function of the number n of days since the video was first shown.