# Problem 245: Coresilience
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) =  1p − 1. Find the sum of all
composite integers 1 < n ≤ 2×1011, for which C(n) is a unit fraction.