Yan Cui

I help clients go faster for less using serverless technologies.

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#### Problem

It is possible to write ten as the sum of primes in exactly five different ways:

7 + 3

5 + 5

5 + 3 + 2

3 + 3 + 2 + 2

2 + 2 + 2 + 2 + 2

What is the first value which can be written as the sum of primes in over five thousand different ways?

#### Solution

// generate all prime numbers under <= this max let max = 1000 let mutable primeNumbers = [2] // only check the prime numbers which are <= the square root of the number n let hasDivisor n = primeNumbers |> Seq.takeWhile (fun n' -> n' <= int(sqrt(double(n)))) |> Seq.exists (fun n' -> n % n' = 0) // only check odd numbers <= max let potentialPrimes = Seq.unfold (fun n -> if n > max then None else Some(n, n+2)) 3 // populate the prime numbers list for n in potentialPrimes do if not(hasDivisor n) then primeNumbers <- primeNumbers @ [n] let isPrime n = if n = 1 then false else not(hasDivisor(n)) // implement the coin change algorithm let rec count n m (coins:int list) = if n = 0 then 1 else if n < 0 then 0 else if (m <= 0 && n >= 1) then 0 else (count n (m-1) coins) + (count (n-coins.[m-1]) m coins) let answer = let tuple = Seq.unfold (fun state -> Some(state, state+1)) 10 |> Seq.map (fun n -> (n, primeNumbers |> Seq.filter (fun n' -> n' < n) |> Seq.cache)) |> Seq.filter (fun (n, l) -> count n (Seq.length l) (Seq.toList l) > 5000) |> Seq.head fst tuple

Yet another twist to problem 31 and problem 76, this time the coin change algorithm will be supplied with only prime numbers less than *n*.

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