**Setup.** Let casisno generate a color: black or red with equal probability.

Let client try to guess the color. If guess is correct - he earns 1 coin from casino, if not - he gives one to casino. If he loses all his coins he stops the game.

Assume that client comes to casino with just 1 coin, while casino has infinite number of coins.

**"Paradox" and informal question**
A small paradox is that casino does not win, while clients do lose. The cheating is that win and loss are in different senses (look below). Nevertheless it is somewhat contrintuitive for me, and some others, so the *informal question* is - can you give an intuitive exlanation.

**Formal questions. Part A. Client's loss quantitatively.**

**(1)** A numerical experiment suggests that probabilty of client to lose behaves like
$P_{loss}(T) = 1 - .8/\sqrt T $ (approximately). $T$ is "time" - number of times we play. What is true formula ? Can one explain in simple way at least this $\sqrt T$ ?

**(2)** Average time to lose seems to behave like $ .8 \sqrt T $. (Averaging is taken over all "loss-trajectories").
What is true formula ? Can one explain in simple way at least this $\sqrt T$ ?
Numerically I see $.8$ in both formulas - is it correct ?

**Theory Background** What I call "client's loss" is usually called "hitting time for a random walk". For $T=\infty$ the distribution of hitting time is known to be Levy distribution (thanks to D. Zare for comment). However this fact does not seem to imply the answer to my question, since I am interested in finite "T", while result is about $T=\infty$.

**Part B. Casino does not win.**

We can cosider the casino profit (number of coins it earned/lossed from the client) after time "T". It is a random variable.

**"Fact"** E(Profit) = 0, for all "T". ("E" is expectation value).

I think it follows from the arguments One can earn nothing on the Brownian motion, true ?, but if someone can propose very simple argument, I would be very thankful.

**"Paradox"** To conclude we see that sooner or later clients will leave all their coins at casino. So it may seems that casino should win. But "fact" above says that expectation value of casino's profit is equal to zero.
There is something contrintuitive, is not it ?

PS

I guess the questions are simple for experts in probability, but I do not think it is reason not to post it here. This is completely rewritten version of previos question.

**Matlab code**

`tStart = tic; %T = 5000; tVec = 500:500:10000; cnt = 0; for T= tVec cnt = cnt + 1; HitCount = 0; HitTime = 0; for trial = 1:100000 %v=2*randi(2,T)-3; % generate random walk's increments - vector of length "T" with random +1 and -1 s = 0; for l=1:T s = s + (2*randi(2,1)-3); % generate random walk %s = sum(v(1:l)); if (s<= -1 ) %check does not Hites "-1" HitCount = HitCount + 1; HitTime = HitTime + l; break; % calculate how many times it Hites out of many trials end; end; end; resultingProbability(cnt) = HitCount/ trial; averageHitTime(cnt) = HitTime/ HitCount; fprintf(1,'HitHitTime/sqrt(T) =%f', averageHitTime(cnt)/sqrt(T)); fprintf(1,'(1- HitProb)*sqrt(T) =%f, \n', averageHitTime(cnt)/sqrt(T)); fprintf(1,'Passed Secs %f \n', toc(tStart) ); end; loglog(tVec, averageHitTime ); figure loglog(tVec, 1-resultingProbability );`