# random walk with reflecting barriers [closed]

Consider a random walk on the line 1,...,d. You start at point 1. At each step you flip a coin: heads means go left, tails means go right. If you're at 1 and get a heads, just stay where you are (same if you're at d and get a tails). Also known as random walk with reflective barriers.

P(x,y) = 1/2 |x-y|=1 or x=y=1 or x=y=d and =0 otherwise

Trying to get an expression for mean number of steps to reach d given that you start in 1, using conditioning on next move.

I tried this but not sure if its correct. Let Sx be the expected number of steps need to go to x+1 given that you start in x. S0=1, Sx=(1/2) + (1/2) Sx-1 + (1/2) Sx and so,

Sx = 1+ Sx-1

## closed as off-topic by Anthony Quas, Stefan Kohl, Yemon Choi, Chris Godsil, Douglas ZareJun 15 '14 at 0:13

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• "MathOverflow is for mathematicians to ask each other questions about their research. See Math.StackExchange to ask general questions in mathematics." – Anthony Quas, Stefan Kohl, Yemon Choi, Chris Godsil, Douglas Zare
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• This question is not about research level mathematics - you might try math.stackexchange.com – Anthony Quas Jun 14 '14 at 16:46