A question that often comes up in math assessments is something like this:
What is the units digit for the power 3 ^ 2012?
For everyone who, like me, doesn’t know what a units digit is: it’s the last
digit of an integer number in decimal representation, the one that contributes
the least to the overall value of the number. For example, in 815
, 5
would
be the units digit.
Tricky thing is, when asked the above question they usually want you to do this
without using a calculator. And in most cases a calculator wouldn’t really
help. Most times, it shows the result to be something like 9,288904458×10⁹⁵⁹
.
Hmm, not helping.
But being programmers, let’s use our computer to solve this:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 

Alright, the answer is 1
. Post done, see you next time!
But OK, let’s see if there is more to it. We have been asked to calculate this without using a calculator or a computer, so there must be an easier way. Besides, what if your programming language were not as helpful as Ruby here and would not support big integers out of the box?
1


Ouch. Sure, I do know about BigInteger
, but let’s for one moment assume that
we did not have such a tool at our hands. What are we really interested in? The
units digit. As it turns out, there is a mathematical tool that expresses just
that. Saying that we want to have the units digit of the number is equivalent
to saying that we want the result of taking the number modulo 10. It’s a handy
feature of the decimal system: whenever we write down a number, let’s say
815
, what this really means in the decimal system is
8 * 100 + 1 * 10 + 5 * 1
or, equivalently
8 * 10² + 1 * 10¹ + 5 * 10⁰
This is also called the
“10adic”
representation of the number 815
. If we take this mod 10
, then all the
terms that are multiples of 10^i
where i > 0
cancel out and we’re left
with 5, which is our units digit:
8 * 10² + 1 * 10¹ + 5 * 10⁰ mod 10
= (8 * 10² mod 10) + (1 * 10¹ mod 10) + (5 * 10⁰ mod 10)
= 0 + 0 + 5
= 5
We are allowed to pull in the mod 10
to each multiplication by the laws of
modular arithmetic, and
each modular multiplication with a factor of 10^i
with i > 0
is 0
since
10
divides 10^i
in those cases. Let’s apply this to our original problem:
1


Sweet, but this still doesn’t help us in case we lack support for large integers. What can we do there?
A first solution
Well, we can again use some modular arithmetic. We can compute
3^2012 mod 10
in the obvious way by first computing 3^2012
and then taking
the result mod 10
or we can reduce each intermediate step directly:
3 ^ 2012 mod 10
= (3 * 3 * ... * 3) mod 10
= (...(((3 * 3 mod 10) * 3 mod 10) * 3 mod 10) * ... ) * 3 mod 10
Translated to Ruby code, this looks like
1 2 3 

Great, and it even works when you can’t use large integer support! So are we done yet? Not quite. For starters, the solution just presented is still far from being done easily without the help of a computer. Sure, it’s manageable to do 2012 multiplications and taking the remainder each time, but come on, that’s a little annoying, isn’t it? On the other hand, the above algorithm works fine if you are operating on relatively small numbers. But once the numbers grow larger, you will notice how the computation will take longer and longer:
1 2 3 

Analyzing our first solution
So, why is it slow? To analyze the algorithm further, let’s first recall some
of the tools that are usually used when analyzing such algorithms. Typically,
running times are given with respect to the length of the binary representation
of the numbers involved. That is, if we compute x^y mod 10
, we would define
k := ceil(log_2(x)) + 1 # length of the binary representation of x
l := ceil(log_2(y)) + 1 # length of the binary representation of y
Our current solution does y
multiplications of the form
r = r * 20122013 % 10
, and this is simply too much. Although y
times
doesn’t look too bad at first, we should note that this number is
exponentially large compared to l
, and this means that we got ourselves
an exponentialtime algorithm with respect to k and l. It doesn’t matter
too much whether x
is large here or not, the following with x := 1
still takes about the same amount of time:
1 2 

Improving our solution
Clearly, our goal has to be reducing the number of multiplications first before we can go any further. If you think of cryptographic algorithms like RSA or DSA, they require huge modular exponentiations, so they’d better use some efficient algorithm to compute them. The venerable Handbook of Applied Cryptography is always a good source for these types of problems. In Ch.14, we are presented with some efficient algorithms for modular exponentiation and we will use the “righttoleft” variant of the famous “square and multiply” method.
1 2 3 4 5 6 7 8 9 10 11 

It produces the correct result, and if we time it, it takes a mere
real 0m0.015s
instead of the 2s it took with our first algorithm.
Analyzing the SquareandMultiply algorithm
Let’s see why this new algorithm is so much faster than our previous one. There
are exactly l
squarings of the form (base * base) % 10
as we divide the
exponent by two in each step  which is equivalent to shifting it to the right
one bit, and we can only do this l
times before it becomes 0
. Next, we have
to perform a multiplication of the form (result * base) % 10
whenever there is
a 1
bit in the exponent, and this can happen l
times in the worst case (all
1
s). Counting yields that we have at most 2l
multiplications plus at most
2l
modular reductions % 10
, as well as l
divisions of the exponent by 2
.
Since a modular reduction by 10
takes O(k * 4) = O(k)
time (modular
reduction a mod n
is O(size(a) * size(n))
), and dividing the exponent by 2
is
also linear in l
at most, the running time is governed by the multiplications.
Each multiplication (result * base)
is roughly O(k²)
or, slightly better,
O(k^log_2(3))
if we use
Karatsuba multiplication. In
total, this gives us a running time of O(l * k^log_2(3))
. This means we have
found a algorithm that is polynomial in l
and k
which is clearly better than
the exponential algorithm we had before.
SquareandMultiply on a piece of paper
It’s even efficient enough that we could use it to solve our initial problem of
computing the units digit of 3^2012
:
result  base  exponent  exponent % 2 == 1 
1  3  2012  no 
1  9  1006  no 
1  1  503  yes 
1  1  251  yes 
1  1  125  yes 
1  1  62  no 
1  1  31  yes 
1  1  15  yes 
1  1  7  yes 
1  1  3  yes 
1  1  1  yes 
1  1  0  no 
Not much going on here as the result stays 1
throughout the algorithm, but you
can easily convince yourself that it would work even if the result were different
than 1
 let’s have a look at the computation of 7^7
, where the units digit is
a 3
:
result  base  exponent  exponent % 2 == 1 
1  7  7  yes 
7  9  3  yes 
3  1  1  yes 
3  1  0  no 
Not too bad after all. We could stop here, having found an algorithm that we can use to solve our initial problem. But whenever designing algorithms, our conscience is constantly nagging us with the ubiquitous question:
Can we do better?
While the SquareandMultiply algorithm is a major improvement over the first algorithm, it being polynomial still means that eventually there are numbers large enough to give it a hard time.
1 2 3 4 5 6 7 8 9 

Running the above takes roughly seven seconds on my computer. Don’t even dare running it with the first algorithm.
You may have noticed that although the article’s title has mentioned Number
Theory, we haven’t seen much of it yet. What is it that this “Units Digit”
problem has in common with Number Theory? Well, let’s look at what happens
if we take consecutive powers of the base in our initial problem, 3
:
3 ^ 1 = 3
3 ^ 2 = 9
3 ^ 3 = 27
3 ^ 4 = 81
3 ^ 5 = 243
3 ^ 6 = 729
3 ^ 7 = 2187
3 ^ 8 = 6561
...
Look closer at the units digits. Pattern, anyone? The sequence is 3, 9, 7, 1
and then it repeats itself, over and over again. Try it  you won’t find a
number in the sequence that doesn’t follow this pattern. Interesting.
If the initial problem would have been
What is the units digit of 5^{2012?}
I bet many of you would have muttered 5
immediately. It seems more anchored
in our brains that for any power of 5
, there is only one outcome for the
units digit  5
. What about this question:
What is the units digit of 2011^{2012?}
It’s gotta be 1
, right? Then again,
What is the units digit of 25^{2012?}
Ahhhm, 5
?
Reducing the general problem to the numbers 09
This leads us to our first important observation: The units digit of a power
is only dependent on the units digit of the base. This implies that
it simply doesn’t matter if you look at 3^2012
or at 2013^2012
 the
units digit will be the same in both cases. Why is this so? If we use the
decimal representation for the base again, we get that
x ^ y mod 10
= (x_0 * 10^0 + x_1 * 10^1 + x_2 * 10^2 + ... + x_n * 10^n) ^ y mod 10
= (x_0^y + a_1 * 10^1 + a_2 * 10^2 + ... + a_n * 10^(n * y)) mod 10
= x_0^y mod 10
where the a_i
s are coefficients that we derive from applying a generalized
version of the laws for
binomial coefficients.
It tells us that the sum of our decimal representation raised to the y
and
then taken mod 10
will only rely on the units digit raised to y
, all
other terms contain some power of 10
and therefore cancel out.
To get an intuition for how we arrived at this fact, let’s have a look at the penandpaper method of multiplying two numbers that we learned in school:
815 * 423 =

326000
16300
2445

344745
We immediately notice that the units digit of the product relies only on the
units digits of the two factors. 5
* 3
is 15
, so the result will have
a units digit of 5
as well. Now think of a power like 2013^2012
. If we write
it out, it looks like this:
2013^2012 = 2013 * 2013 * ... * 2013
The intermediate result of the first two factors will only rely on the units digits of the two factors, as we have just seen. But then , the result of that computation multiplied again with the third factor in the chain will also just rely on the units digits and so on and so forth  we have just outlined an inductive proof of our claim.
The fact that the units digit of a power only depends on the units digit of
its base is huge since it tells us that the “Units Digit” problem for arbitrary
powers boils down to knowing the units digits for powers of the numbers that
are smaller than 10
. We just cut down the problem for infinitely many powers
to powers of the numbers zero through nine. Not bad.
Solving for the number ‘3’ using the repeating pattern
Now our intuition told us that there is most certainly a repeating pattern for
powers of 5
and 1
 the resulting power’s unit digit is always a 5
or
1
respectively. In our example for the number 3
, we also saw that there is
a repeating cycle of exactly 4
numbers, and we now know that we only need to
solve the problem for the numbers zero through nine.
How convenient would it be if there were a repeating cycle for any of those
numbers, right? And it’s here where our final piece of Number Theory fits in.
In our example for 3
, “repeating units digits after every fourth power” can
be expressed mathematically as
3^4 congruent 1 mod 10
This is incredibly powerful because it lets us immediately derive the units digits of higher powers, too:
3^21 mod 10
= (3 ^ 4 * 3 ^ 4 * 3 ^ 4 * 3 ^ 4 * 3 ^ 4) * 3 mod 10
= 1 * 1 * 1 * 1 * 1 * (3 mod 10)
= 3
Put in simple terms: We just need to know the remainder of 21 mod 4
, 1
, to
reduce our problem down to solving 3 mod 10
to get to our units digit. Compare
this to our solution using SquareandMultiply: all we have to do now is to compute
r:= exponent % 4
and solve the problem 3^r mod 10
. In terms of our initial
problem:
3^2012 mod 10
= 3 ^ (4 * 503) * 1 mod 10
= 1 * 1 mod 10
= 1
Awesome, right? Now that’s something we can really do immediately on a piece of
paper. So how about the rest of the numbers? 0
as a special case is of course
guaranteed to cycle because 0 * x = 0
for arbitrary x
. But is there an
equally useful pattern for the rest? Our intuition told us so far that at least
for 0
, 1
and 5
there should be, but how about the rest?
Enter Euler’s Theorem
Euler’s Theorem tells us that
x^φ(10) mod 10 is congruent to 1
where φ(10)
are the numbers between 1
and 10
that are coprime to 10
(their gcd with 10
is equal to 1
). This holds whenever x
is coprime
to 10
as well. The numbers smaller than 10
that are coprime to 10
are
exactly 1
,3
,7
and 9
. So φ(10) = 4
and this proves that a^4 mod 10
is congruent to 1
for a
equal to 1
,3
,7
or 9
. Or in other words:
We have proven that the units digits for 1
,3
,7
and 9
will repeat after
at most four consecutive powers.
What about the rest of the numbers, 0
,2
,4
,5
,6
and 8
? As φ
also
determines the order of the
multiplicative group of integers modulo n,
consulting Lagrange’s theorem
shows us that the order of any element must divide the order of that group.
Therefore the order of any element within the group can only be as high as
the total group order itself. The order of an element here is the smallest
number i
where a^i congruent to 1 mod n
for a
an element of the group.
The group order for a multiplicative group of integers modulo n
is the
number of elements it contains, which happens to be exactly φ(n)
. In our
case, where the group order is 4
(= φ(10)
), this means that the order
of any element cannot be more than 4
as well. In plain English: There is
no number smaller than 10
where it takes more than four consecutive powers
until the units digit of those powers enter a cycle.
We conclude that each of the units digits of powers of 1..9
repeat after
at most four consecutive powers.
Coming up with a constanttime algorithm
We can use this knowledge now to construct a constanttime algorithm for our
initial problem. With all that we know now, we know it suffices to memorize
the units digits of the first 4 powers (in the order a^4
, a^1
, a^2
and
a^3
) of the numbers zero through nine to solve the “Units Digit” problem for
arbitrary powers:
i: [i^4 mod 10, i^1 mod 10, i^2 mod 10, i^3 mod 10]

0: [0, 0, 0, 0]
1: [1, 1, 1, 1]
2: [6, 2, 4, 8]
3: [1, 3, 9, 7]
4: [6, 4, 6, 4]
5: [5, 5, 5, 5]
6: [6, 6, 6, 6]
7: [1, 7, 9, 3]
8: [6, 8, 4, 2]
9: [1, 9, 1, 9]
Using these in a precomputed array, here is finally a constanttime algorithm as promised:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

Let’s try out the number that brought the SquareandMultiply algorithm to its knees:
1 2 3 4 5 6 7 8 9 

Ridiculous. That’s all you got?
Astute readers might point out that this is not really a
constanttime algorithm because of the modular reductions in it and they
are right in doing so. Can we fix this? Turns out we can: we could require
a string representation of our base, and we would simply pick only its
units digit and transform that into an integer to get rid of the first
modular reduction. Regarding the second reduction modulo 4
, this should
really be a very efficient operation already if our (byte code) compiler
is worth its money. Taking a number % 4
is equivalent to masking it with
3
on the binary level. Why? 4
is a power of two, 2²
, and therefore the
remainder is simply the last two digits of the binary representation 
which we get by masking the number with number & 0x03
because the binary
representation of 3
is 11
 the mask we need to just keep the last two
binary digits.
To illustrate this, let’s look at the assembly that GCC generates for such a computation.
1 2 3 4 5 6 7 8 

And here is the assembly from gcc S mod4.c
(only the relevant parts):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

As you can see, it’s doing what we expected, “ANDing” the input with 3
to
get the result.
Final words
I hope you liked the subject as much as I did when exploring it. I found it really entertaining how a question that might look boring on the surface wound up involving so many different topics, finally even leading to Number Theory. Applying more and more Math, we were able to get from an exponential solution to a polynomial solution all the way down to a constanttime algorithm.
When I was first exposed to Number Theory, the guy told us
While Number Theory is considered by many to be the most beautiful subject of Mathematics, be warned that you will probably never find a useful application for it in your future lives.
Apart from cryptography, he was right about this. It is beautiful and interesting to know more about the origins and relations of our numbers, but I had never ever needed Euler’s theorem for anything else than RSA so far. I was really happy that after such a long time I was finally able to see it applied in this context again. Of course, you might want to argue about how “useful” this exercise was exactly in the end. But if you have made it this far, then at least you seemed to have liked it, too!