Posts Tagged ‘totatives’

Programming Praxis – Divisors And Totatives

November 26, 2010

In today’s Programming Praxis exercise,our goal is to make a small library to compute the following of a number: the divisors, the sum of the divisors, the number of divisors, the totatives and the totient. Let’s get started, shall we?

Some imports:

import Data.List
import Data.Numbers.Primes
import Data.Ratio

Calculating the divisors of a number could be done the compact but naive way (divisors n = filter ((== 0) . mod n) [1..n]), but let’s go with the slightly longer but more efficient version of multiplying every combination of prime factors.

divisors :: Integral a => a -> [a]
divisors = nub . sort . map product . subsequences . primeFactors

Getting the sum of divisors is trivial.

divisorSum :: Integral a => a -> a
divisorSum = sum . divisors

For the amount of divisors we’ll ignore the trivial method (length . divisors) and go with the more efficient algorithm.

divisorCount :: Integral a => a -> Int
divisorCount = product . map (succ . length) . group . primeFactors

The totatives can be be trivially calculated using trial division.

totatives :: Integral a => a -> [a]
totatives n = filter ((== 1) . gcd n) [1..n]

For the totient there is also a better way than the trivial one (length . totatives):

totient :: (Integral a, Integral b) => a -> b
totient n = floor . product $ n % 1 : [1 - 1 % f | f <- nub $ primeFactors n]

As usual, some tests to see if everything is working properly:

main :: IO ()
main = do print $ divisors 60 == [1,2,3,4,5,6,10,12,15,20,30,60]
          print $ divisorSum 60 == 168
          print $ divisorCount 60 == 12
          print $ totatives 30 == [1,7,11,13,17,19,23,29]
          print $ totient 30 == 8
          print $ totient 60 == 16

Looks like it is, and at five lines of code I’d call this a pretty small library 🙂 Fortunately, the non-trivial algorithms don’t add much in the way of length. Sure, the lines are a little longer, but they’re all still one-liners. I’d say it’s worth it for the improved speed.