A friend asked comments about some websites he found, which promise to double your money deposit after you’ve joined them. The system works like this: you deposit N dollars to the site, and when new members join into the site, you can expect your payment to be doubled within some days.

The system in question seemed to be somekind of a variation of the snailmail pyramid schemes like chain letters, or a Ponzi scheme (named after Charles K. Ponzi), but in a different disguise. I decided to analyze the system a little bit as an intellectual diversion for that evening.
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