“One of the simplest models in probability theory. A description of an urn model is as follows: Consider some vessel — an urn — with black and white balls. One ball is drawn at random from the urn, and then it is returned to the urn together with cc balls of the same colour as the ball drawn and dd balls of the other colour. After mixing the balls in the urn, the procedure is repeated a certain number of times. It is assumed that initially the urn contains a>0a>0 white and b>0b>0 black balls. The numbers cc and dd, the parameters of the urn model, may also be negative.” – Encycopedia of Mathmatics
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