[Speculative] Tried replacing phiFunction with linear sieve that can calculate mobius as well.

[Chillee]

Not sure I like it. There might be other better/more general ways of calculating multiplicative functions. On the other hand, if we're including mobius inversion I think it makes sense to have a way of calculating the mobius function.

[simonlindholm]

Seems kinda non-obvious how to change. I think a useful primitive might be a sieve that computes the smallest (or largest) prime that divides every number. Then it would be easy to compute phi:

rep(a,2,n) {
  int p = smallestPrimeDivisor[a], b = a / p;
  phi[a] = phi[b] * (p - (smallestPrimeDivisor[b] != p));
}

This may be too easy to code by hand to be worth including though.