Problem 245

We shall call a fraction that cannot be cancelled down a resilient fraction.
Furthermore we shall define the resilience of a denominator, R(d), to be the ratio of its proper fractions that are resilient; for example, R(12) = 411.

The resilience of a number d > 1 is then

d - 1
, where φ is Euler's totient function.
We further define the coresilience of a number n > 1 as C(n)
n - φ(n)

n - 1
The coresilience of a prime p is C(p)

p - 1

Find the sum of all composite integers 1 <n ≤ 2×1011, for which C(n) is a unit fraction.

These problems are part of Project Euler and are licensed under CC BY-NC-SA 2.0 UK