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From my perspective, while the problem itself is not as complicated as I expected, it was figuring out how to design an eligible efficient algorithm that was the actual problem.

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import scala.math.BigInt

object ProjectEuler245Coresilience { val eulerTotient:BigInt => BigInt = n => { var result: BigInt = n var i: BigInt = 2 var tmp: BigInt = n while (i * i <= tmp) { if (tmp % i == 0) { while (tmp % i == 0) { tmp /= i } result -= result / i } i = i + 1 } if (tmp > 1) result -= result / tmp result }

val coresilience: BigInt => BigInt = n =>{ a= n-eulerTotient(n) (a / a.gcd(n-1)) }

val sumOfUnitFractions:BigInt => BigInt = n => { var sumOf: BigInt = 0 for (i: BigInt <- BigInt(2) to n) { if (coresilience(i) == 1) { sumOf += i } } sumOf }

var a: BigInt = 0

def main(args: Array[String]): Unit = { val stdin = scala.io.StdIn val test = stdin.readInt println(sumOfUnitFractions(test)) } }

Here is my solution with Scala and I couldn't go further optimization. Any help??