# Multiplicative Functions

## Definition

In number theory, a multiplicative function is an arithmetic function f(n) of a positive integer n that satisfies:

• f(1) = 1
• For every co-prime pair of integer a and b: f(ab)=f(a)f(b)

### Examples

• Id_k(n): the power functions, defined by id_k(n) = n^k for any number k. As special cases we have:
• \epsilon(n): the unit function, defined by \epsilon(n) = 1 if n = 1 and 0 otherwise.
• gcd(n,k): the greatest common divisor of n and a fixed integer k, as a function of n
• \sigma_k(n): the divisor function, which is the sum of the k-th powers of all the positive divisors of n. As special cases we have:
• \sigma_0(n) = d(n): the number of positive divisors of n
• \sigma_1(n) = \sigma(n), the sum of all the positive divisors of n
• \phi(n): Euler’s totient function
• \mu(n): the Mobius function
• \mu(n) = 0 if n is not square-free
• \mu(n) = 1 if n has even numbers of prime factors
• \mu(n) = -1 if n has odd numbers of prime factors

### Transitive property

Given two multiplicative functions f(x) and g(x), we have:

• Their product h(x) = f(x) g(x) is also multiplicative.
• Their Dirichlet convolution (f * g)(n) = \sum_{d | n}{f(d) * g(n/d)} is also multiplicative (more about Dirichlet convolution below).
Specifically, if f(x) is multiplicative, h(n) = \displaystyle \sum_{d | n}{f(d)} is also multiplicative.

## Calculation using the linear prime sieve

Let’s consider this problem:

• Given n \le 1e7 and a function f(x), calculate the value of f(i) for every integer i in the segment [1,n].

If we can prove that f(x) is multiplicative and can calculate f(p^k) in O(1) for any prime number p and integer k, we can utilize the linear prime sieve to solve this problem in O(n).

If you don’t know what the linear prime sieve is, read about it here before continuing.

Consider the original implementation of linear prime sieve:

const int N = 10000000;
int lp[N + 1];
vector<int> pr;

for (int i = 2; i <= N; ++i) {
if (lp[i] == 0) {
lp[i] = i;
pr.push_back(i);
}
for (int j = 0; j < (int)pr.size() && pr[j] <= lp[i] && i * pr[j] <= N; ++j)
lp[i * pr[j]] = pr[j];
}

For simplicity, let denote pr[j] as p. There are 3 cases:

• i is prime (lp[i] = 0). In this case, we determine the value of f(i) directly.
• p < lp[i]. Since lp[i] is the smallest prime factor of i, i and p must be co-prime.
We can assign f(ip) = f(i)f(p)
• p = lp[i]. This is the most complicated case since i and p are not co-prime.
Most of the time, a simple relationship between them exists. For example, with Euler’s totient function, \phi(ip) = \phi(i) * p.

Note: Don’t forget to initialize f = 1 (very important).

Here is the code to calculate both Euler’s totient function and Mobius function:

const int N = 10000000;
int lp[N + 1], phi[N + 1], mu[N + 1];
vector<int> pr;

phi = mu = 1;
for (int i = 2; i <= N; ++i) {
if (lp[i] == 0) {
// first case
lp[i] = i;
phi[i] = i - 1;
mu[i] = -1;
pr.push_back(i);
}
for (int j = 0; j < (int)pr.size() && pr[j] <= lp[i] && i * pr[j] <= N; ++j) {
lp[i * pr[j]] = pr[j];
if (pr[j] < lp[i]) {
// second case
phi[i * pr[j]] = phi[i] * phi[pr[j]];
mu[i * pr[j]] = mu[i] * mu[pr[j]];
} else {
// third case
phi[i * pr[j]] = phi[i] * pr[j];
mu[i * pr[j]] = 0;
}
}
}

In case we can’t find any relationship between i and p, we can work around by using an auxiliary array cnt[i] which denotes the power of p in i.

As \dfrac{i}{p^{cnt[i]}} and p^{cnt[i] + 1} are co-prime, we have:
f(ip) = f\left (\dfrac{i}{p^{cnt[i]}} \right ) * f\left (p^{cnt[i] + 1} \right )

To avoid unnecessary recalculation of p^{cnt[i]}, we will store it as pw[i].

Refer to sum of GCD application below for sample implementation.

## Dirichlet convolution

Dirichlet convolution is a way to generate a new function from two functions.

Formally, the Dirichlet convolution of two functions f(x) and g(x) is:
(f * g)(n) = \sum_{d_1 * d_2=n}{f(d_1) * g(d_2)}

Another representation is:
(f * g)(n) = \sum_{d | n}{f(d) * g(n/d)}

It has a very nice property: the Dirichlet convolution of two multiplicative functions is also a multiplicative function.

#### Proof

Consider any two co-prime numbers a and b. Every divisor d of ab can be uniquely reprensented as rs, with r|a, s|b and gcd(r,s) = 1.

\begin{array}{rl} (f * g)(ab) &= \sum_{r|a, s|b}{f(rs) g(ab/rs)} \\\\ &= \sum_{r|a, s|b}{f(r) f(s) g(a/r) g(b/s)} \\\\ &= \sum_{r|a}{f(r) g(a/r)} \sum_{s|b}{f(s) g(b/s)} \\\\ &= (f * g)(a) (f * g)(b) \end{array}

Therefore, (f * g) is multiplicative.

### Example

To illustrate the power of Dirichlet convolution, we will use it to prove the divisor function is multiplicative.

Let f(n) = n^k and g(n) = 1. Obviously f and g are multiplicative functions.
(f * g)(n) = \sum_{d | n}{f(d) * g(n/d)} = \sum_{d | n}{d^k} = \sigma_k(n)
Therefore, the divisor function is multiplicative.

You can find more examples on Wikipedia.

## Applications

### Sum of GCD

Given a number n, find the sum of GCDs of all distinct unordered pairs that can be formed with integers from 1 to n (unordered means that (i, j) and (j, i) are considered the same).

We will calculate h(x) = \displaystyle \sum_{1 \le i \le x}{gcd(x, i)}.

The answer for the problem would be \displaystyle \sum_{1 \le i \le n}{\left (h(i)-i \right )}.

h(x) can be rewritten as: h(x) = \displaystyle \sum_{d | x}{d * count(d)} = \sum_{d | x}{d * \phi(x / d)}.

Here count(d) is the number of pairs (x, i) with GCD equal to d. For every such pair, we have: gcd(x / d, i / d) = 1.

Therefore, count(d) = \phi(x / d).

Let’s use Dirichlet convolution with f(x) = x and g(x) = \phi(x). We get h(x) is multiplicative.

The only thing left now is how to calculate h(p^k) for any prime number p and integer k.

\begin{array}{rl} h(p^k) &= \displaystyle \sum_{d | p^k}{d * \phi(p^k / d)} \\\\ &= p^k + \displaystyle \sum_{0 \le i \le k-1}{p^i(p^{k-i}-p^{k-i-1})} \\\\ &= p^k + k * (p^k-p^{k-1}) \\\\ &= (k + 1) * p^k-k * p^{k-1} \end{array}

Sample code:

const int N = 10000000;

int lp[N + 1], cnt[N + 1], pw[N + 1];
long long h[N + 1], ans[N + 1];
vector<int> pr;

h = 1; ans = 0;
for (int i = 2; i <= N; ++i) {
if (lp[i] == 0) {
// first case
lp[i] = pw[i] = i;
cnt[i] = 1;
h[i] = 2 * i - 1;
pr.push_back(i);
}
ans[i] = ans[i - 1] + h[i] - i;
for (int j = 0; j < (int)pr.size() && pr[j] <= lp[i] && i * pr[j] <= N; ++j) {
lp[i * pr[j]] = pr[j];
if (pr[j] < lp[i]) {
// second case
cnt[i * pr[j]] = 1;
pw[i * pr[j]] = pr[j];
h[i * pr[j]] = h[i] * h[pr[j]];
} else {
// third case
cnt[i * pr[j]] = cnt[i] + 1;
pw[i * pr[j]] = pw[i] * pr[j];
// tmp = h[pr[j] ^ (cnt[i] + 1)]
ll tmp = 1LL * (cnt[i * pr[j]] + 1) * pw[i * pr[j]] - 1LL * cnt[i * pr[j]] * pw[i];
h[i * pr[j]] = h[i / pw[i]] * tmp;
}
}
}

### Number of co-prime pairs in a set

Given a set S of numbers. Find the number of co-prime pairs in this set.

We will use inclusion-exclusion principle and Mobius function to solve this problem.

For simplicity, denote a pair (x, y) divisible by k if both x and y are divisible by k.

Using inclusion-exclusion principle, the answer is:

+ number of pairs divisible by 1
- number of pairs divisible by 2
- number of pairs divisible by 3
- number of pairs divisible by 5
...
+ number of pairs divisible by 2 * 3
+ number of pairs divisible by 2 * 5
+ number of pairs divisible by 3 * 5
...
- number of pairs divisible by 2 * 3 * 5
...

Let cnt_k be the number of multiples of k in S.

The answer is: \displaystyle \sum{\dfrac{cnt_i * (cnt_i-1)}{2} * \mu(i)}.

Some problems require adding/removing numbers in a set. To add/remove x, we only need to update cnt_k for k divides x. Complexity of each operation is O(\sigma_0(x)).

#### Arbitrary GCD

To count pairs (x, y) with gcd(x, y) = g for any g, take all multiple of g in S and divide them by g to create a new set. We move back to the original problem: count co-prime pairs in the new set.

#### Number of relative primes

Instead of counting pairs, we want to know how many numbers in S are relative prime to x.

Still using inclusion-exclusion principle and Mobius function, the formula is:
\sum_{d | x}{cnt_d * \mu(d)}

When we update cnt for some number x, we consider many unnecessary divisors of x: non-square-free divisors, for which Mobius function is 0.

For example, with x = 16 = 2^4, we have 5 divisors (1, 2, 4, 8, 16), but only two of them count (1 and 2). The rest have Mobius value of 0.

Let the prime factorization of x be p_1^{e_1} \cdot p_2^{e_2} \cdots p_k^{e_k}. We will only consider divisors of type p_1^{a_1} \cdot p_2^{a_2} \cdots p_k^{a_k} where a_i \in \left \{ 0; 1 \right \}. Now it’s clear that we can use bitmask, each bit corresponds to one prime factor.

But how many bits will we need to consider? The number with most prime factors is 2 * 3 * 7 * \cdots. Keep multiplying until it’s larger than the maximum of x.

For example, if maximum of x is 5e5, we only need to consider at most 6 bits, since 2 * 3 * 5 * 7 * 11 * 13 * 17 = 510510 is larger than 5e5.

In addition, we don’t need to pre-calculate Mobius function. Count number of bits set, if it’s odd, Mobius value is -1, otherwise, it’s 1.

## Practice Problems

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