Step aside

Brilli the ant decides to have a walk along a 1D line. For each step he takes, he would either walk forward or backward by a distance of 1 with equal probability. What is the expected number of steps Brilli would take before he reaches a distance of 2016 away from where he started?


The answer is 4064256.

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1 solution

Mark Hennings
Nov 27, 2016

Let E n E_n be the expected number of steps that Brilli must take to reach a distance N N from his starting point, given that he is currently a distance of n n from his starting point. Then we have the equations E n = 1 + 1 2 ( E n + 1 + E n 1 ) 1 n N 1 E_n \; = \; 1 + \tfrac12(E_{n+1} + E_{n-1}) \hspace{2cm} 1 \le n \le N-1 together with the boundary conditions E 0 = 1 + E 1 E N = 0 E_0 \; = \; 1 + E_1 \hspace{2cm} E_N \; = \; 0 Solving these equations, we obtain E n = N 2 n 2 0 n N E_n \; = \; N^2 - n^2 \hspace{2cm} 0 \le n \le N and hence E 0 = N 2 E_0 = N^2 . With N = 2016 N=2016 , this makes the answer 201 6 2 = 4064256 2016^2 = \boxed{4064256} .

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