Question: Let Sn D X1 C C Xn where the Xi are independent with EXi D 0 and var .Xi / D 2.
Let Sn D X1 C C Xn where the Xi are independent with EXi D 0 and var .Xi / D 2.
(a) Show that S2 n n2 is a martingale.
(b) Let D min fn W jSnj > ag. Use Theorem 5.13 to show that E a2=2. For simple random walk 2 D 1 and we have equality.
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