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) = ⁴⁄₁₁.

The resilience of a number d 1 is then

φ(d) 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)

=

1 p - 1

.

Find the sum of all composite integers 1 n 210¹¹, for which C(n) is a unit fraction.

Note: the upper limit has been changed recently. Check out that you use the right upper limit.