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Given a positive integer n output the integers a and b (forming reduced fraction a/b) such that:

Formula a/b = product of k=1 to n: (p_k^2 - 1) / (p_k^2 + 1)

Where pk is the k th prime number (with p1 = 2).


1   -> 3, 5
2   -> 12, 25
3   -> 144, 325
4   -> 3456, 8125
5   -> 41472, 99125
15  -> 4506715396450638759507001344, 11179755611058498955501765625
420 -> very long

Probabilistic prime checks are allowed, and it's ok if your answer fails due to limitations in your language's integer type.

Shortest code in bytes wins.

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Can we also output 3.0 instead of 3? – Adnan Mar 23 at 16:00
@AandN I guess... Make sure your program is correct for all inputs though, and does not suffer from floating point errors for big inputs. – orlp Mar 23 at 16:06
Can we output a and b as a rational type? – Alex A. Mar 24 at 0:31
@AlexA. Only if the output clearly shows both integers. – orlp Mar 24 at 0:48
@SamYonnou Those already exist, but abusing native number types to trivialize a problem is one of the loopholes that are forbidden by default. – Dennis Mar 25 at 3:46

13 Answers 13

Mathematica, 32 bytes


An unnamed function that takes integer input and returns the actual fraction.

This uses the fact that (p2-1)/(p2+1) = 1-2/(p2+1). The code is then golfed thanks to the fact that Mathematica threads all basic arithmetic over lists. So we first create a list {1, 2, ..., n}, then retrieve all those primes and plug that list into the above expression. This gives us a list of all the factors. Finally, we multiply everything together by applying Times to the list, which can be golfed to 1##&.

Alternatively, we can use Array for the same byte count:

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1-2=1, right? – CalculatorFeline Mar 23 at 18:07
@CatsAreFluffy Yeah (-1 actually), but 1-2/x ≠ -1/x. ;) – Martin Ender Mar 23 at 18:08
@Range@±~Array~ – CalculatorFeline Mar 23 at 18:37

Python 2, 106 bytes

from fractions import*
while n:b=P%k>0;n-=b;F*=1-Fraction(2*b,k*k+1);P*=k*k;k+=1
print F

The first and fourth lines hurt so much... it just turned out that using Fraction was better than multiplying separately and using gcd, even in Python 3.5+ where gcd resides in math.

Prime generation adapted from @xnor's answer here, which uses Wilson's theorem.

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M, 9 bytes


Try it online!


Meet M!

M is a fork of Jelly, aimed at mathematical challenges. The core difference between Jelly and M is that M uses infinite precision for all internal calculations, representing results symbolically. Once M is more mature, Jelly will gradually become more multi-purpose and less math-oriented.

M is very much work in progress (full of bugs, and not really that different from Jelly right now), but it works like a charm for this challenge and I just couldn't resist.

How it works

RÆN²‘İḤCP  Main link. Argument: n

R          Range; yield [1, ..., n].
 ÆN        Compute the kth primes for each k in that range.
   ²‘      Square and increment each prime p.
     İ     Invert; turn p² + 1 into the fraction 1 / (p² + 1).
      Ḥ    Double; yield 2 / (p² + 1).
       C   Complement; yield 1 - 2 / (p² + 1).
        P  Product; multiply all generated differences.
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Is ÆN the only M-specific operator? Also Melly – CalculatorFeline Apr 11 at 16:54
None of these operators are specific to M. The difference is that M calculates a fraction, while Jelly calculates a floating point number. – Dennis Apr 11 at 16:55

Ruby, 122 77 65 bytes

Thanks to Sherlock for shaving off 10 bytes.


Defines an anonymous function that takes a number and returns a Rational.

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Jelly, 14 13 bytes


Try it online! Thanks to @Dennis for -1 byte.

R                       Range [1..n]
 ÆN                     Nth prime
   ²                    Square
    µ                   Start new monadic chain
     ’ż‘                Turn each p^2 into [p^2-1, p^2+1]
        P               Product
         µ              Start new monadic chain
          ÷             Divide by...
           g/           Reduce GCD
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Pyth, 26 25


Try it here or run the Test Suite.

1 byte saved thanks to Jakube!

Pretty naive implementation of the specifications. Uses the spiffy "new" (I have no idea when this was added, but I've never seen it before) P<neg> which returns whether the positive value of a negative number is prime or not. Some of the mapping, etc can probably be golfed...

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PARI/GP, 33 bytes


Alternate version (46 bytes):


Non-competing version giving the floating-point (t_REAL) result (38 bytes):

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Julia, 59 42 bytes


This is an anonymous function that accepts an integer and returns a Rational with BigInt numerator and denominator.

We begin by generating the list of prime numbers less than 2n2 and selecting the first n elements. This works because the nth prime is always less than n2 for all n > 1. (See here.)

For each p of the n primes selected, we square p using elementwise power (.^2), and construct the rational 2 / (p + 1), where 2 is first converted to a BigInt to ensure sufficient precision. We subtract this from 1, take the product of the resulting array of rationals, and return the resulting rational.

Example usage:

julia> f = n->prod(1-big(2).//-~primes(2n^2)[1:n].^2)
(anonymous function)

julia> f(15)

Saved 17 thanks to Sp3000!

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Convex, 28 bytes

Convex is a new language that I am developing that is heavily based on CJam and Golfscript. The interpreter and IDE can be found here. Input is an integer into the command line arguments. Indexes are one-based. Uses the CP-1252 encoding.


You may or may not consider this answer to be competing since I was working on a few features that this program uses before the challenge was posted, but the commit was made once I saw this challenge go out.

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MATL, 18 bytes


Try it online!

Fails for large inputs because only integers up to 2^52 can be accurately represented internally.


:     % implicitly take input n. Generate range [1,...,n]
Yq    % first n prime numbers
2^    % square
tqp   % duplicate. Subtract 1. Product
wQp   % swap. Add 1. Product
Zd    % gcd of both products
1M    % push the two products again
h     % concatenate horizontally
w/    % swap. Divide by previously computed gcd. Implicitly display
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Mathematica, 45 bytes


Primes? Fractions? Mathematica.

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Haskell, 53 bytes

Anonymous function, 53 characters:


Try it here (note: in standard GHCi you need first to make sure Data.Ratio and Data.List are imported):

λ (scanl(*)1[1-2%(p*p+1)|p<-nubBy(((>1).).gcd)[2..]]!!) 5
41472 % 99125
:: Integral a => Ratio a

Haskell's list indexing !! is 0-based. (___!!) is an operator section, forming an anonymous function so that (xs !!) n == xs !! n.

It's four bytes less to generate the whole sequence:

λ mapM_ print $ take 10 $     -- just for a nicer output
    scanl(*)1[1-2%(n*n+1)|n<-[2..],all((>0).rem n)[2..n-1]]
1 % 1
3 % 5
12 % 25
144 % 325
3456 % 8125
41472 % 99125
3483648 % 8425625
501645312 % 1221715625
18059231232 % 44226105625
4767637045248 % 11719917990625
:: IO ()
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Seriously, 25 bytes


Outputs a\nb (\n is a newline). Large inputs will take a long time (and might fail due to running out of memory) because prime generation is pretty slow.

Try it online!


,r                         push range(input)
  `PªD;⌐k`M                map:
   P                         k'th prime
    ª                        square
     D                       decrement
      ;                      dupe
       ⌐                     add 2 (results in P_k + 1)
        k                    push to list
           ┬               transpose
            `π`M           map product
                i│         flatten, duplicate stack
                  g;)      push two copies of gcd, move one to bottom of stack
                     @\    reduce denominator
                       )\  reduce numerator
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