Polya's Urn (Discrete Martingale)

An urn contains b black and r red balls. A ball is drawn at random and is put back along with c more identical balls of the color drawn.

Let Y_n be the proportion of black balls after n similar draws.

Show that Y_n is a discrete martingale with E(Y_n)=b/(b+r)

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If anyone helps me, I will be thankful.

Selin