Serge Gautier posed me the following question:
Various autopilot software (subways, cars, planes...) use error detecting systems, to avoid being fooled by accidental data corruption. For example, an integer X can be coded with two fields (x, r), with r = kx mod A for some integers k and A. X is valid if (kx  r) mod A = 0. Suppose x is altered by an error (but not r) and becomes (x + e): this error won't be detected if k(x + e)  r mod A = ke mod A = 0, which amounts to e mod A = 0 if we wisely take k coprime with A. So the general assumption is that such systems have probability 1/A of not detecting a random error. Serge is wondering what this means regarding the distribution of e, the "random errors", over the naturals, if such assumptions were to hold regarding all possible choices of A>1. Let p(A) be the probability that e is not detected in modulo A, given that it is polled from a specific distribution, D, over the naturals. What distributions will satisfy p(A)=1/A exactly? Can an adversary design a distribution that will always satisfy p(A)≪1/A? Can an adversary design a distribution that will always satisfy p(A)≫1/A? To make these questions more concrete, let f(x) and g(x) be two monotone strictly decreasing functions over the naturals such that for all x, 0<f(x)<1 and 0<g(x)<1, and when x tends to infinity f and g both tend to zero. Answer all of the following questions with proof.

List of solvers:Gaoyuan Chen (2 September 17:21)JJ Rabeyrin (9 September 15:59) RaduAlexandru Todor (12 September 08:08) Luke Pebody (14 September 16:26) Oscar Volpatti (30 September 04:15) 
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