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Least divisor function

Function

definiendum ld:N+{1}Prime number
definiendum ld(n):=min(divisors(n))

Code

Haskell
divides :: Integral a => a -> a -> Bool
divides d n = rem n d == 0
ld :: Integral a => a -> a
ld n = ldf 2 n
 
ldf :: Integral a => a -> a -> a
ldf k n | divides k n = k 
      	| k^2 > n     = n
	| otherwise   = ldf (k+1) n

using Set of divisors function:

divides :: Integral a => a -> a -> Bool
divides d n = rem n d == 0

Theorems

If n isn't a prime, then n divided by the least divisor is some number bigger than ld(n) and hence

ld(n)2n

Subset of

Surjective function

Requirements

Prime number, Set of divisors function

Code reference

Set of divisors function