# Computing $\sum_{k=1}^n (a^k \bmod m)$

I would like to find a closed form solution for

$$\sum_{k=1}^n (a^k \bmod m)$$ $$0 0$$

Note that the mod operator is within the brackets.

If a closed form solution does not exist, I'm interested in good approximations, and how I should have gone about concluding that a closed form solution can't be found.

Solutions that require restrictions on $$a$$ and $$m$$ are of interest too.

# Background

$$a^k \bmod m$$ is a very simple hashing function. It gives the $$k$$th output of the simplified linear congruential generator PRNG $$X_{k+1} = (a X_k \bmod m)$$. Large values of $$a$$ and $$m$$ are typically seen. A common example is $$a=16807, m=2^{31}-1$$. I'm looking for a way to sum the first $$n$$ pseudorandom outputs. My only requirement is that it be visually random, so values of $$a$$ and $$m$$ (and even small modifications to the formula) are flexible. An ideal solution would work for any $$a$$ and $$m$$, but that is not essential.

# What I've tried

There's a very obvious basic linear approximation of a sum of evenly distributed random values given by $$k$$ times the mean value. This isn't good enough for my case as I need to capture the visually random nature of the values.

The main trick that I know for dealing with mod is to isolate the behaviour of a single period and then sum it over each. I'm struggling to apply that here for this nonlinear case.

I started by tackling some simplified similar problems. If we remove the mod operator, this is just a geometric series which is obviously trivial. I tried tackling the simpler $$\sum_{k=1}^n(k \bmod m)$$ and was able to get a result by isolating the period:

$$\left\lfloor \frac a n \right\rfloor \sum_{i=1}^{m-1}i + \sum_{i=1}^{n \bmod m}i$$

By a similar process, I thought it might be possible to isolate the geometric series and sum over the periods. While $$a^k < m$$ the geometric series gives the answer. This can be written as

$$\sum_{k=1}^{\lfloor\log_a m \rfloor} a^k$$

I don't know how to take this further. I tried using $$\lfloor\log_a jm\rfloor$$ to construct a sum of sums along the following lines, but that introduces awkward floor functions as limits and I don't know what to do with that at all.

$$\sum_{j=1}^l {\sum_{k=\lfloor\log_a jm\rfloor}^{\lfloor\log_a (j+1)m\rfloor}{a^k}}$$

I think those limits are a little bit wrong actually, but I'm not worried about nailing them down unless I know how to handle their form.

I also tried a different approach, by rewriting the mod function using floor.

$$\sum_{k=1}^n (a^k - m \left\lfloor \frac{a^k}{m} \right\rfloor)$$

$$\sum_{k=1}^n a^k - m\sum_{k=1}^n \left\lfloor \frac{a^k}{m} \right\rfloor$$

From here I just face the same difficulties with the floor part as I did above. It feels like there may not be a closed form solution available, but I don't know how I would go about realising that and knowing to stop looking.

• ${a^{n+1} -1 \over a-1} \mod m$. – copper.hat Jun 19 at 19:44
• Oh, I had a dumb moment there, sorry @copper.hat. That's just the geometric series, right? I explained why that's not a good enough approximation for my case. – Sam Jun 19 at 19:56
• Unless $a=1$ you must have a computational mistake. – copper.hat Jun 19 at 19:56
• It is exact, there is no approximation. I am unable to ind an explanation of why it does not work above. – copper.hat Jun 19 at 19:59
• My misunderstanding. I read the question properly. Sorry about that. – copper.hat Jun 19 at 20:00

## 1 Answer

$$\newcommand{\ord}{\mathop{\rm ord}\nolimits}$$

As a technical note about notation: $$(x \ \mathrm{mod}\ m)$$ is an infinite set of integers and sums of infinite sets are more infinite sets. You could unambiguously indicate what you want by $$\sum_{k=1}^n f(a^k \ \mathrm{mod}\ m) \text{,}$$ where $$f$$ takes a residue class $$x \ \mathrm{mod}\ m$$ to its least non-negative member. (Note that other choices are possible. A not entirely rare choice is the member with least magnitude. Note that this choice has an ambiguity when the modulus is even, so a full specification requires resolving whether to produce the positive or negative member when both have the same magnitude.)

How do we compute $$s(a,m,n) = \sum_{k=1}^n f(a^k \ \mathrm{mod}\ m) \text{?}$$ Since we can reduce the modular arithmetic if $$\gcd(a,b) > 1$$, let us assume $$\gcd(a,m) = 1$$. (If $$\gcd(a,m) = b > 1$$, then we should instead study $$s(a/b, m/b,n)$$.) Let $$\phi$$ be the Euler totient function and $$\ord_m a$$ be the multiplicative order of $$a$$ modulo $$m$$. By Euler's theorem, $$\ord_m a$$ divides $$\phi(m)$$, so as long as we can factor $$\phi(m)$$, a binary search can find the multiplicative order of $$a$$ modulo $$m$$.

The powers of $$a$$ modulo $$m$$ are periodic, with period $$\ord_m a$$. So we apply the division algorithm to determine how many whole periods and how much is left when we partition $$n$$ into pieces of size $$\ord_m a$$. In particular, let $$q$$ and $$r$$ be such that $$q \geq 0$$, $$0 \leq r < \ord_m a$$ and $$n = q \ord_m a + r \text{.}$$ We find that $$n$$ is $$q$$ full periods of the powers of $$a$$ with a remainder of $$r$$ more powers of $$a$$.

Let $$p(a,m) = \sum_{k=1}^{\ord_m a} f(a^k \ \mathrm{mod}\ m) \text{.}$$ Then $$s(a,m,n) = q \, p(a,m) + \sum_{k=1}^r f(a^k \ \mathrm{mod}\ m) \text{.}$$

Examples (timings appear to the right): $$\ord_{2^{31}-1}16\,807 = 2\,147\,483\,646$$. \begin{align*} n& & s(16\,807, n, 2^{31} - 1)& \\ 1& & 16\,807& \\ 2& & 282\,492\,056& \\ 3& & 1\,905\,142\,129& \\ 4& & 2\,890\,085\,787& \\ 5& & 4\,034\,194\,717& \\ 10& & 9\,529\,300\,043& \\ 15& & 13\,441\,838\,169& \\ 20& & 17\,004\,096\,180& \\ 40& & 42\,026\,616\,212& \\ 60& & 64\,128\,301\,397& \\ 80& & 87\,098\,290\,857& \\ 100& & 111\,330\,854\,817& \\ 10^5& & 107\,435\,233\,385\,977& & (0.1 \,\mathrm{s})& \\ 10^6& & 1\,073\,806\,376\,451\,147& & (1.2 \,\mathrm{s})& \\ 10^7& & 10\,737\,818\,730\,605\,039& & (14 \,\mathrm{s})& \end{align*} $$\ord_{2^{31}-1}2 = 31$$.
\begin{align*} n& & s(2, n, 2^{31} - 1)& \\ 1& & 2& \\ 2& & 6& \\ 3& & 14& \\ 4& & 30& \\ 5& & 62& \\ 10& & 2046& \\ 15& & 65534& \\ 20& & 2\,097\,150& \\ 40& & 2\,147\,484\,669& \\ 60& & 3\,221\,225\,469& \\ 80& & 4\,295\,491\,580& \\ 100& & 6\,442\,451\,195& \\ 10^5& & 6\,925\,701\,870\,437& & (70 \,\mu\mathrm{s})& \\ 10^6& & 69\,273\,527\,484\,932& & (57 \,\mu\mathrm{s})& \\ 10^7& & 692\,735\,276\,946\,410& & (65 \,\mu\mathrm{s})& \\ 10^8& & 6\,927\,365\,633\,427\,248& & (62 \,\mu\mathrm{s})& \\ 10^9& & 69\,273\,664\,924\,010\,478& & (63 \,\mu\mathrm{s})& \end{align*} Note that, as a matter of coding,

• If $$\ord_m a$$ is not too large, you should probably cache $$p(a,m)$$ and the table of values of $$\sum_{k=1}^r f(a^k \ \mathrm{mod}\ m)$$ for $$r =0, 1, \dots, (\ord_m a)-1$$. Then $$s(a,m,n)$$ is a quotient and remainder calculation to get $$q,r$$ and then $$q\,p(a,m)$$ is a simple multiplication and $$\sum_{k=1}^r f(a^k \ \mathrm{mod}\ m)$$ is a table lookup.
• If $$\ord_m a$$ is too large to store a full table of sums for each possible remainder, store a table of $$a^{2^k}\ \mathrm{mod}\ m$$ for $$k = 1, \dots, \lfloor \log_2(\ord_m a) \rfloor$$ and use exponentiation by squaring (with operations modulo $$m$$). (The timings in the table above did not use this method, although $$\ord_{2^{31} - 1}16\,807 = 2\,147\,483\,646$$ is an excellent candidate, requiring only $$31$$ squares of $$a$$ modulo $$m$$ be stored.)
• There should be a method, similar to the Fast Fourier Transform to decompose the remainder sum into blocks by which bits are set to $$1$$ in $$k$$ and accumulate the powers in $$r \log r$$ time rather than $$r^2$$ time, but I have not thought through the details of such a method.
• This looks like a fantastic answer, thanks. I'll need to get around to testing it out before I accept it because there's some stuff to get my head around here, but I'll come back and accept it if it works, which it looks like it will. If someone were able to find a closed form solution that was more readily amenable to further algebraic manipulation that would be preferable, but otherwise this is looking good. I can work with it and hopefully take these ideas forward to some slightly different scenarios. – Sam Jun 20 at 17:17
• The use of exponentiation by squaring to tackle the sum is the main thing that didn't click for me until you said it, and which was holding me back. That's a trick I should have in my toolbox. – Sam Jun 20 at 17:19