The formula is given without explanation. You can rederive it without too much trouble.<p>You can't use the common formula for compound interest because users are assumed to send invites only once. Compound interest would assume they were sending invites continuously over the cycles.<p>If you look at the number of users added each cycle (the top row) you can see that it doubles each time. It's given by c0 times K^i, where c0 is the initial number of customers, K is the virality coefficient (2 in this case), and i is the i-th cycle.<p>Adding each of the terms up to i to the original c0 gives the total number of customers after i cycles. So you get a sum:<p><pre><code> sum over i from 0 to N of (c0 * K^i)
</code></pre>
which using an exponential sum formula (<a href="http://mathworld.wolfram.com/ExponentialSumFormulas.html" rel="nofollow">http://mathworld.wolfram.com/ExponentialSumFormulas.html</a>) gives:<p><pre><code> c0 * ((1-K^(N+1)) / (1 - K))
</code></pre>
Multiply by -1/-1<p><pre><code> c0 * ((K^(N+1) - 1) / (K - 1))
</code></pre>
and N is the number of cycles (given by t/ct in the slides)<p><pre><code> c0 * ((K^(t/ct+1) - 1) / (K - 1))</code></pre>
> A more pervasive example is the “Sent from my iPhone/iPad” signature at the bottom of every email you send from your iPhone or iPad. Yes, even Apple is using viral tactics (actually, I believe BlackBerry started that with “Sent using BlackBerry”).<p>As noted in "Founders at Work", Hotmail used this tactic with a lot of success. I don't know if they were the first to try it, but they were definitely the first widespread example.
Am I wrong in thinking that G+ attempts all of these things and yet still struggles, or am I in the wrong circles? Or is it that Google has yet to reach a tipping point with their product?