public long GetAnswerTo70()
{
const int limit = 10000000;
var sieve = new CoprimeSieve(limit);
return Sequences.Range(2, limit).MinByWithPostCondition(
n => (double)n / sieve.Phi(n),
n => IsPermutation(n, sieve.Phi(n)));
}
public long GetAnswerTo72()
{
const int limit = 1000000;
var sieve = new CoprimeSieve(limit);
return Sequences.Range(2, limit).Sum(n => sieve.Phi(n));
}
public long GetAnswerTo69()
{
const int limit = 1000000;
var sieve = new CoprimeSieve(limit);
return Sequences.Range(2, limit).AsParallel().MaxBy(n => (double)n / sieve.Phi(n));
}