# Writing rational numbers as ratio of factorials of primes

Note: this challenge has been posted on the sandbox.

# Introduction

This challenge is inspired by 2009 Putnam B1, a problem in an undergraduate mathematics competition. The problem is as follows:

Show that every positive rational number can be written as a quotient of products of factorials of (not necessarily distinct) primes. For example,

# Challenge

Your challenge is to take a pair of relatively prime positive integers, representing the numerator and denominator of a positive rational number (or just the rational number itself) as input, and output two lists (or arrays, etc.) of prime numbers so that the inputted rational number is equal to the ratio of the product of the factorials of the primes in the first list to the product of the factorials of the primes in the second list.

### Notes

• There may not be any primes that contained both in the first list and in the second list; however, a prime may appear as many times as one wishes in either list.
• The inputs can be assumed to each be (nonstrictly) between 1 and 65535; however, it cannot be assumed that the factorials of the numbers you will need to output will be in this range.

# Example Input and Output

Here are examples of legal inputs and outputs.

input=>output
10,9 => [2,5],[3,3,3]
2,1 => [2],[]
3,1 => [3],[2]
1,5 => [2,3,2],[5]     (elements of a list may be in any order)
3,2 => [3],[2,2]
6,1 => [3],[]


The inputs (2,2), (0,3), (3,0), (3,6) and (1,65536) are illegal inputs (i.e. your program doesn't need to behave in any particular way on them). Here are some examples of illegal outputs:

1,2 => [2],[2,2] (2 is in both returned lists)
5,2 => [5],[2,4] (4 is not prime)
2,1 => [2],[1] (1 is not prime either)
3,2 => [3],[2] (3!/2! = 3, not 3/2)


# Scoring

This is , so the lowest score in bytes wins!

• Does some sort of minimally reduced rational need to be given in the event there are multiple ways of expressing the answer? For example 10/9 = [2*5]/[3*3] = [(2!/1!) * (5!/4!)] / [(3!/2!) * (3!/2!)] = [2! * 5! * 2! * 2!] / [3! * 3! * 1! * 4!] = (2! * 2! * 2! *5!) / (3! * 3! * 4!). – Digital Trauma Oct 6 '17 at 21:45
• @DigitalTrauma No; however, 4 is not prime, so the second one would not be valid. I believe (and can write up a proof in the question if you'd like) that any representation is unique. – Carl Schildkraut Oct 6 '17 at 22:02
• Is it okay to take input as the fraction 10/9 rather than a pair of numbers 10 and 9? – Misha Lavrov Oct 6 '17 at 22:03
• @MishaLavrov Sure. I'll edit the question to reflect that. – Carl Schildkraut Oct 6 '17 at 22:03
• @CarlSchildkraut Thanks - yes that helps - I thought I was missing something – Digital Trauma Oct 6 '17 at 22:04

# 05AB1E, 5453484640353332 28 bytes

[D¿÷Z#DÓ€gZD<ØŠQ*DˆR!*]¯øεʒĀ


Try it online! Edit: Saved 2 bytes thanks to @ASCII-only. Saved 1 2 3 4 bytes thanks to @Emigna. (I only need to save one more and I'm down to half my original byte count!) Explanation:

[       Begin an infinite loop
D¿÷     Reduce to lowest terms
Z#      Exit the loop if the (largest) value is 1
DÓ€g    Find the index of the largest prime factor of each value
Z       Take the maximum
D<ØŠ    Convert index back to prime and save for later
Q       Convert to an pair of which value had the largest prime factor
*       Convert to an pair with that prime factor and zero
Dˆ      Save the pair in the global array for later
R!*     Multiply the other input value by the factorial of the prime
]       End of infinite loop
¯ø      Collect all the saved primes
εʒĀ     Forget all the saved 0s

• I love "emotional" scripts - ¦D – RedClover Oct 8 '17 at 5:54
• 46 bytes – ASCII-only Oct 8 '17 at 10:18
• 39 bytes – Mr. Xcoder Oct 9 '17 at 11:53

# Mathematica, 175177169154 108 bytes

Join@@@Table[x[[1]],{s,{1,-1}},{x,r@#},x[[2]]s]&@*(If[#==1,1,{p,e}=Last@(r=FactorInteger)@#;p^e#0[p!^-e#]]&)


Try it online!

### How it works

This is the composition of two functions. The first, which ungolfs to

If[# == 1,
1,
{p,e} = Last[FactorInteger[#]];
p^e * #0[p!^-e * #]
]&


is a recursive function for actually computing the desired factorization. Specifically, given a rational input x, we compute the primes whose factorials should be in the numerator and denominator, and return the fraction with all of those primes multiplied together. (For example, on input 10/9 = 2!*5!/(3!*3!*3!), we return 10/27 = 2*5/(3*3*3).)

We do this by dealing with the largest prime factor at every step: if pe occurs in the factorization of x, we make sure p!e occurs in the factorial-factorization, and recurse on x divided by p!e.

(Earlier, I had a more clever strategy that avoids large numbers by looking at the previous prime number before p, but Mathematica can handle numbers as big as 65521! easily, so there's no point. The old version you can find in the history is much faster: on my computer, it took 0.05 seconds on inputs that this version handles in 1.6 seconds.)

The second function turns the output of the first function into lists of primes.

Join @@@
Table[x[[1]],
{s,{1,-1}},
{x,FactorInteger[#]},
x[[2]]*s
]&


For s=1 (positive powers) and s=-1 (negative powers), and for each term {prime,exponent} in the factorization r@#, we repeat the prime number prime exponent*s many times.

# Noncompeting version with 109 62 bytes

If[#==1,∇1=1,{p,e}=Last@FactorInteger@#;(∇p)^e#0[p!^-e#]]&


Same as above, but instead of giving output as a list, gives output as an expression, using the ∇ operator (because it has no built-in meaning) as a stand-in for factorials. Thus, an input of 10/9 gives an output of (∇2*∇5)/(∇3)^3 to represent (2!*5!)/(3!)^3.

This is shorter because we skip the second part of the function.

+2 bytes: the assignment f=First has to be done in the right place to keep Mathematica from getting upset.

-8 bytes: fixed a bug for integer outputs, which actually made the code shorter.

-15 bytes: FactorInteger returns sorted output, which we can take advantage of.

-46 bytes: we don't actually need to be clever.

## Python 2, 220202195 183 bytes

g=lambda a,b:a and g(b%a,a)or b;n,d=input();m=c=()
while n+d>2:
t=n*d;f=p=2
while t>p:
if t%p:p+=1;f*=p
else:t/=p
if n%p:c+=p,;n*=f
else:m+=p,;d*=f
t=g(n,d);n/=t;d/=t
print m,c


Try it online! Edit: Saved 18 25 bytes thanks to @Mr.Xcoder. Saved 12 bytes thanks to @JonathanFrech.

• 202 bytes – Mr. Xcoder Oct 7 '17 at 6:41
• You can shorten it even more in Python 2, since you can replace multiple spaces with tabs in the indentation – Mr. Xcoder Oct 7 '17 at 6:43
• – Jonathan Frech Oct 7 '17 at 16:52
• – Jonathan Frech Oct 7 '17 at 16:55