Chain Letters (Kettenbriefe):



References:
 
-
  letter chains (Kettenbriefe), 
  Stochastik in der Schule 12:3 (1992) 37

-
  Probability models of pyramid or chain letter systems demonstrating
  that their promotional claims are unreliable,
  Operations Research 32 (1984) 527-536

- David J. Thuente;
  Chain Letters: A Poor Investment Unless...,
  Two-Year College Mathematics Journal 13:1 (Jan. 1982) 28-35

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