[Link to PDF (Latex Formatted)](https://www.dropbox.com/s/ny5nx5swg8bueth/divisors.pdf?dl=0)↵
↵
**Topic** : Counting Divisors of a Number↵
↵
**Pre Requisites** : Basic Maths , $O(\sqrt[]N)$ Factorisation , Primality testing↵
↵
**Motivation Problem** :↵
↵
There are $T$ test cases. Each test case contains a number $N$. For each test case , output the number of factors of $N$.↵
↵
$1 <= T <= 10$↵
↵
$1 <= N <= 10^{18}$↵
↵
_Note_ : $1$ and $N$ are also treated as factors of the number $N$.↵
↵
**Solution** :↵
↵
**Naive Solution** — _$O(\sqrt[]N)$ Factorisation_↵
↵
The most naive solution here is to do the $O(\sqrt[]N)$ factorisation. But as we can see, it will surely receive the Time Limit Exceeded verdict. You may try to compute the prime numbers till required range and loop over them , but that too exceeds the usual limit of $10^8$ operations. Some optimisations and heuristics may allow you to squeeze through your solution, but usually at the cost of a few TLE's.↵
↵
Supposing you fit in the above description and cannot think of anything better than the naive solution, I would like to present to you a very simple algorithm which helps you count the number of divisors in $O(N^{\frac{1}{3}})$, which would be useful for this kind of questions. **This algorithm only gives us the count of factors, and not the factors itself.**↵
↵
Firstly, let's do a quick recap of how we obtain the $O(\sqrt[]N)$ algorithm for any number $N$ :↵
↵
We write $N$ as product of two numbers $P$ and $Q$.↵
↵
$P*Q = N , where \hspace{1mm} P <= Q$↵
↵
$Maximum \hspace{1mm} value \hspace{1mm} of \hspace{1mm} P = \sqrt[]{N}$↵
↵
Looping over all values of $P$ gives us the count of factors of $N$.↵
↵
↵
Before you move forward to the next section, it would be useful if you try to come up with an algorithm that finds the count of factors in $O(N^{\frac{1}{3}})$. As a hint, the way of thinking is same as that of getting to the $O(\sqrt[]N)$ algorithm.↵
<br/><br/>↵
↵
Counting factors in $\mathbf{O(N^{\frac{1}{3}})}$ Factorisation↵
-------------------------------------------↵
↵
Let's start off with some maths to reduce our $O(\sqrt[]N)$ factorisation to $O(N^{\frac{1}{3}})$ for counting factors :↵
↵
We write $N$ as product of three numbers $P$, $Q$ and $R$.↵
↵
$P*Q*R = N , where \hspace{1mm} P <= Q <= R$↵
↵
$Maximum \hspace{1mm} value \hspace{1mm} of \hspace{1mm} P = N^{\frac{1}{3}}$↵
↵
We can loop over all prime numbers in range $[2,N^{\frac{1}{3}}]$ and try to reduce $N$ to it's prime factorisation, which would help us count the number of factors of $N$.↵
↵
To know how to calculate divisors using prime factorisation, [click here](http://mathschallenge.net/library/number/number_of_divisors).↵
↵
We will split our number $N$ into two numbers $X$ and $Y$ such that $X*Y=N$. Further, $X$ contains only prime factors in range $[2,N^{\frac{1}{3}}]$ and $Y$ deals with higher prime factors ($ > N^{\frac{1}{3}}$). Thus, gcd($X$ , $Y$) = 1. Let the count of divisors of a number $N$ be denoted by the function $F(N)$. It is easy to prove that this function is multiplicative in nature, i.e., $F(m*n) = F(m)*F(n)$, if gcd($M$,$N$) = 1. So, if we can find $F(X)$ and $F(Y)$, we can also find $F(X*Y)$ or $F(N)$ which is the required quantity.↵
↵
For finding $F(X)$, we use the naive trial division to prime factorise $X$ and calculate the number of factors. Once this is done, we have $Y=N/X$ remaining to be factorised. This may look tough, but we can see that there are only three cases which will cover all possibilities of $Y$ :↵
↵
1. **$\mathbf{Y}$ is a prime number** : $F(Y)$ = $2$.↵
2. **$\mathbf{Y}$ is square of a prime number** : $F(Y)$ = $3$.↵
3. **$\mathbf{Y}$ is product of two distinct prime numbers** : $F(Y)$ = $4$.↵
↵
We have only these three cases since there can be at max two prime factors of $Y$. If it would have had more than two prime factors, one of them would surely have been $ <= N^{\frac{1}{3}}$, and hence it would be included in $X$ and not in $Y$.↵
↵
So once we are done with finding $F(X)$ and $F(Y)$, we are also done with finding $F(X*Y)$ or $F(N)$.↵
↵
**Pseudo Code** :↵
↵
↵
~~~~~↵
N = input()↵
primes = array containing primes till 10^6↵
ans = 1↵
for all p in primes :↵
if p*p*p > N:↵
break↵
count = 1↵
while N divisible by p:↵
N = N/p↵
count = count + 1↵
ans = ans * count↵
if N is prime:↵
ans = ans * 2↵
else if N is square of a prime:↵
ans = ans * 3↵
else if N != 1:↵
ans = ans * 4↵
~~~~~↵
↵
↵
↵
↵
Checking for primality can be done quickly using Miller Rabin. Thus, the time complexity is $O(N^{\frac{1}{3}})$ for every test case and hence we can solve our problem efficiently.↵
↵
At this point, you may think that in a similar way, we can reduce this to $O(N^{\frac{1}{4}})$ by handling some cases. I have not thought much on it, but the number of cases to be handled are high since after trial division $N$ could be factorised into one, two or three primes. This is easy enough to code in contest environment, which is our prime objective.↵
↵
This trick is not quite commonly known and people tend to make bugs in handling the three cases. A problem in regionals which uses this trick directly :↵
↵
[Problem F | Codeforces Gym](http://codeforces.net/gym/100753)↵
↵
You can try also this technique on problems requiring $\sqrt[]{N}$ factorisation for practice purposes.↵
↵
Hope you found this useful! Please suggest more problems to be added as well as any edits, if required.↵
↵
**Happy Coding!**
↵
**Topic** : Counting Divisors of a Number↵
↵
**Pre Requisites** : Basic Maths , $O(\sqrt[]N)$ Factorisation , Primality testing↵
↵
**Motivation Problem** :↵
↵
There are $T$ test cases. Each test case contains a number $N$. For each test case , output the number of factors of $N$.↵
↵
$1 <= T <= 10$↵
↵
$1 <= N <= 10^{18}$↵
↵
_Note_ : $1$ and $N$ are also treated as factors of the number $N$.↵
↵
**Solution** :↵
↵
**Naive Solution** — _$O(\sqrt[]N)$ Factorisation_↵
↵
The most naive solution here is to do the $O(\sqrt[]N)$ factorisation. But as we can see, it will surely receive the Time Limit Exceeded verdict. You may try to compute the prime numbers till required range and loop over them , but that too exceeds the usual limit of $10^8$ operations. Some optimisations and heuristics may allow you to squeeze through your solution, but usually at the cost of a few TLE's.↵
↵
Supposing you fit in the above description and cannot think of anything better than the naive solution, I would like to present to you a very simple algorithm which helps you count the number of divisors in $O(N^{\frac{1}{3}})$, which would be useful for this kind of questions. **This algorithm only gives us the count of factors, and not the factors itself.**↵
↵
Firstly, let's do a quick recap of how we obtain the $O(\sqrt[]N)$ algorithm for any number $N$ :↵
↵
We write $N$ as product of two numbers $P$ and $Q$.↵
↵
$P*Q = N , where \hspace{1mm} P <= Q$↵
↵
$Maximum \hspace{1mm} value \hspace{1mm} of \hspace{1mm} P = \sqrt[]{N}$↵
↵
Looping over all values of $P$ gives us the count of factors of $N$.↵
↵
↵
Before you move forward to the next section, it would be useful if you try to come up with an algorithm that finds the count of factors in $O(N^{\frac{1}{3}})$. As a hint, the way of thinking is same as that of getting to the $O(\sqrt[]N)$ algorithm.↵
<br/><br/>↵
↵
Counting factors in $\mathbf{O(N^{\frac{1}{3}})}$
-------------------------------------------↵
↵
Let's start off with some maths to reduce our $O(\sqrt[]N)$ factorisation to $O(N^{\frac{1}{3}})$ for counting factors :↵
↵
We write $N$ as product of three numbers $P$, $Q$ and $R$.↵
↵
$P*Q*R = N , where \hspace{1mm} P <= Q <= R$↵
↵
$Maximum \hspace{1mm} value \hspace{1mm} of \hspace{1mm} P = N^{\frac{1}{3}}$↵
↵
We can loop over all prime numbers in range $[2,N^{\frac{1}{3}}]$ and try to reduce $N$ to it's prime factorisation, which would help us count the number of factors of $N$.↵
↵
To know how to calculate divisors using prime factorisation, [click here](http://mathschallenge.net/library/number/number_of_divisors).↵
↵
We will split our number $N$ into two numbers $X$ and $Y$ such that $X*Y=N$. Further, $X$ contains only prime factors in range $[2,N^{\frac{1}{3}}]$ and $Y$ deals with higher prime factors ($ > N^{\frac{1}{3}}$). Thus, gcd($X$ , $Y$) = 1. Let the count of divisors of a number $N$ be denoted by the function $F(N)$. It is easy to prove that this function is multiplicative in nature, i.e., $F(m*n) = F(m)*F(n)$, if gcd($M$,$N$) = 1. So, if we can find $F(X)$ and $F(Y)$, we can also find $F(X*Y)$ or $F(N)$ which is the required quantity.↵
↵
For finding $F(X)$, we use the naive trial division to prime factorise $X$ and calculate the number of factors. Once this is done, we have $Y=N/X$ remaining to be factorised. This may look tough, but we can see that there are only three cases which will cover all possibilities of $Y$ :↵
↵
1. **$\mathbf{Y}$ is a prime number** : $F(Y)$ = $2$.↵
2. **$\mathbf{Y}$ is square of a prime number** : $F(Y)$ = $3$.↵
3. **$\mathbf{Y}$ is product of two distinct prime numbers** : $F(Y)$ = $4$.↵
↵
We have only these three cases since there can be at max two prime factors of $Y$. If it would have had more than two prime factors, one of them would surely have been $ <= N^{\frac{1}{3}}$, and hence it would be included in $X$ and not in $Y$.↵
↵
So once we are done with finding $F(X)$ and $F(Y)$, we are also done with finding $F(X*Y)$ or $F(N)$.↵
↵
**Pseudo Code** :↵
↵
↵
~~~~~↵
N = input()↵
primes = array containing primes till 10^6↵
ans = 1↵
for all p in primes :↵
if p*p*p > N:↵
break↵
count = 1↵
while N divisible by p:↵
N = N/p↵
count = count + 1↵
ans = ans * count↵
if N is prime:↵
ans = ans * 2↵
else if N is square of a prime:↵
ans = ans * 3↵
else if N != 1:↵
ans = ans * 4↵
~~~~~↵
↵
↵
↵
↵
Checking for primality can be done quickly using Miller Rabin. Thus, the time complexity is $O(N^{\frac{1}{3}})$ for every test case and hence we can solve our problem efficiently.↵
↵
At this point, you may think that in a similar way, we can reduce this to $O(N^{\frac{1}{4}})$ by handling some cases. I have not thought much on it, but the number of cases to be handled are high since after trial division $N$ could be factorised into one, two or three primes. This is easy enough to code in contest environment, which is our prime objective.↵
↵
This trick is not quite commonly known and people tend to make bugs in handling the three cases. A problem in regionals which uses this trick directly :↵
↵
[Problem F | Codeforces Gym](http://codeforces.net/gym/100753)↵
↵
You can try also this technique on problems requiring $\sqrt[]{N}$ factorisation for practice purposes.↵
↵
Hope you found this useful! Please suggest more problems to be added as well as any edits, if required.↵
↵
**Happy Coding!**
