# Probability mass function for the first non-increasing sample from a random sequence

Consider a sequence of random numbers drawn IID from some distribution $$g(x)$$. How would I determine the distribution of the value of the first sample from that sequence which is not greater than all previous samples? What about the special case where $$g(x)$$ is uniform over $$[0, 1]$$?

Example: Some example IID sequences drawn from $$U(0,1)$$ are

0.35, 0.29, 0.19, 0.64, ...
^^^^
0.33, 0.35, 0.22, 0.62, ...
^^^^
0.21, 0.01, 0.98, 0.11, ...
^^^^
0.59, 0.77, 0.93, 0.17, ...
^^^^


The first non-monotonic samples are 0.29, 0.22, 0.01, 0.17. I'm interested in the distribution function for these values, either in the general or the $$U(0,1)$$ case.

• Is this a continuous variable in your more general case? (the continuous case is really straightforward; the answer is the same for all continuous distributions) Or do you seek to cover discrete variables? What about discrete/continuous mixtures? – Glen_b Apr 17 '19 at 0:00
• Actually, it looks like the continuous case is completely covered by the answer here (mutatis mutandis in relation to the shift-by-one in the definition of the variable); I will leave this open for the moment (pending clarification on the above question). – Glen_b Apr 17 '19 at 0:29
• – Ben Apr 17 '19 at 1:37
• Note that I'm not asking about the length of the sequence, but rather the value of the first non-monotonic sample. I'm interested in the continuous case only. – ChrisB Apr 17 '19 at 2:21
• Yep, got it now; you want he unconditional distribution of $X_L$ (where $L$ is defined as the position of the first "dip") – Glen_b Apr 17 '19 at 2:54

Let $$X_1,X_2,X_3,... \sim \text{IID } G$$ be your exchangeable sequence of random variables and define $$N \equiv \max \{ n \in \mathbb{N} | X_1 < X_2 < \cdots < X_n \}$$, which is the length of the largest increasing portion at the start of the sequence. You are looking for the distribution of the random variable $$X_{N+1}$$, which is the first value that is not greater than all the previous values. This question is tractable if the distribution $$G$$ is continuous, but it gets much more complicated if the distribution has any discrete part, since that requires dealing with ties. For simplicity, I am going to show you the answer that holds when you have a continuous distribution.

Some preliminary work: We will use $$G$$ to denote the (continuous) distribution function and $$g$$ to denote the corresponding density function. In this related question it was established that the mass function for $$n$$ is:

$$p_N(n) = \mathbb{P}(N=n) = \frac{n}{(n+1)!} \quad \quad \quad \quad \text{for all } n \in \mathbb{N}.$$

Since $$N=n$$ requires that $$X_1< \cdots (and the maximum is invariant to the order) we also have:

$$\mathbb{P}(X_n \leqslant r | N=n) = \mathbb{P}(X_i \leqslant r)^{n+1} = G(r)^{n+1},$$

which gives the corresponding density:

$$p(X_n = r|N=n) = (n+1) g(r) G(r)^n.$$

Finding the target density: Using the above results we have:

\begin{aligned} p(X_{N+1} = x|N=n) &= \int p(X_{n+1} = x | X_n = r, N=n) \ dP(X_{n} \leqslant r | N=n) \\[6pt] &= \int \limits_x^\infty p(X_{n+1} = x | X_n = r, N=n) \cdot p(X_{n} = r | N=n) \ dr \\[6pt] &= \int \limits_x^\infty p(X_{n+1} = x | X_{n+1} \leqslant r) \cdot p(X_{n} = r | N=n) \ dr \\[6pt] &= \int \limits_x^\infty \frac{g(x)}{G(r)} \cdot (n+1) g(r) G(r)^{n} \ dr \\[6pt] &= g(x) \int \limits_x^\infty (n+1) g(r) G(r)^{n-1} \ dr \\[6pt] &= g(x) \Bigg[ \frac{n+1}{n} \cdot G(r)^{n} \Bigg]_{r=x}^{r \rightarrow \infty} \\[12pt] &= \frac{n+1}{n} \cdot g(x) (1 - G(x)^n). \\[6pt] \end{aligned}

Hence, we have:

\begin{aligned} p(X_{N+1} = x) &= \sum_{n=1}^\infty \ p(X_{n+1} = x | N=n) \cdot \mathbb{P}(N=n) \\[6pt] &= \sum_{n=1}^\infty \ \frac{n+1}{n} \cdot g(x) (1 - G(x)^n) \cdot \frac{n}{(n+1)!} \\[6pt] &= g(x) \sum_{n=1}^\infty \ \frac{1 - G(x)^n}{n!} \\[6pt] &= g(x) \sum_{n=0}^\infty \ \frac{1 - G(x)^n}{n!} \\[6pt] &= g(x) ( e - e^{G(x)} ). \\[6pt] \end{aligned}

The corresponding distribution function is:

\begin{aligned} \mathbb{P}(X_{N+1} \leqslant x) &= \int \limits_{-\infty}^x g(r) ( e - e^{G(r)} ) \\[6pt] &= \Bigg[ G(r) \cdot e - e^{G(r)} \Bigg]_{r \rightarrow - \infty}^{r=x} \\[6pt] &= \Bigg[ G(x) \cdot e - e^{G(x)} - (-1) \Bigg] \\[6pt] &= 1 + G(x) \cdot e - e^{G(x)}. \\[6pt] \end{aligned}

Special case - uniform distribution: In the case where we use the standard uniform distribution we have $$g(x) = 1$$ and $$G(x) = x$$ for all $$0 \leqslant x \leqslant 1$$ so we have:

\begin{aligned} p(X_{N+1} = x) &= e - e^{x}. \\[6pt] \end{aligned}

• Thank you! This all looks correct except the equation for 𝑝(𝑋𝑛=𝑟|𝑁=𝑛) -- I believe this is (n+1)g(r)G(r)^n (because X_n is the maximum of n+1 samples in this context, not n samples). This simplifies the overall result to a sum of 1/n! g(x) (1 - G(x)^n). I confirmed this result with Monte Carlo simulation for the uniform case. – ChrisB Apr 17 '19 at 18:17
• and furthermore, for the uniform case, the result simplifies to e - e^x – ChrisB Apr 17 '19 at 18:23
• Well spotted - I have edited the answer to correct this, and that does indeed simplify the mathematics considerably. – Ben Apr 17 '19 at 22:43

@Ben provides most of the solution, here's a summary that fixes a minor error:

The sequence we are interested in consists of N monotonic elements ($$X_1 < X_2 < ... < X_N$$) followed by a non-monotonic element at position N+1. We are interested in $$p(X_{N+1}=x)$$

The PDF for the length of the monotonic sequence at the start of the chain (excluding the first non-monotonic sample) is $$p(N=n) = \frac{n}{(n+1)!}$$

The PDF for the maximum value among the first N+1 elements (which, in this context, is also the value of the element at position N) is $$p(X_n=r | N=n) = (n+1)g(r)G(r)^n$$, where G is the cumulative distribution function corresponding to g.

The distribution for $$X_{N+1}$$ is $$p(X_{N+1} = x) = \sum_{n=1}^{\infty}\int_x^{\infty}{p(X_{N+1}=x|X_N=r, N=n)p(X_n=r|N=n)p(N=n)dr}$$

Using the fact that $$X_{N+1} < X_{N}$$, the first term is equivalent to

$$\sum_{n=1}^{\infty}\int_x^{\infty}{p(X_{N+1}=x|X_{N+1} < r, N=n)p(X_n=r|N=n)p(N=n)dr}$$

Which we can flip around using Bayes' rule:

$$\sum_{n=1}^{\infty}\int_x^{\infty}{\frac{p(X_{N+1}

Finally, plugging in $$g$$ and $$G$$ based on the above identities:

$$\sum_{n=1}^{\infty}\int_x^{\infty} \frac{g(x)}{G(r)}(n+1)g(r)G(r)^n\frac{n}{(n+1)!} dr$$

$$\sum_{n=1}^{\infty}g(x)\frac{1}{n!}\int_x^{\infty}ng(r)G(r)^{n-1} dr$$

$$\sum_{n=1}^{\infty}\frac{g(x)}{n!}[1 - G(x)^n]$$

For the case where $$X \sim U(0,1)$$, $$g(x)=1$$, $$G(x)=x$$, and

$$p(X_{N+1})=\sum_{n=1}^{\infty}\frac{1-x^n}{n!} = e-e^x$$

Which we can confirm against Monte Carlo simulation:

def sample(g = np.random.uniform):
x = top = g()
while x >= top:
top = x
x = g()
return x

def f(x):
return np.e - np.exp(x)

plt.hist([sample() for _ in range(100000)], bins=100, density=True)
x = np.linspace(0, 1, 20)
y = np.array([f(xx) for xx in x])
plt.plot(x, y)