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I have two lists of integers. The first, p, is a list of primes (for example, {2,3,5,7}). The second is a list t of three arbitrary integers (positive, negative, or zero), for example {10,15,24}. You are guaranteed at the outset that each element of t is prime to at least one element of p. I want to produce a list s having the same length as p, where each element of s comes from t (repetitions allowed) and such that CoprimeQ[s[[i]],p[[i]]] is True for each i. So in the above example {15,10,24,15} and {15,10,24,24} would both be acceptable answers.

I can produce a matrix that expresses whether each element of t is prime to each element of p using

Table[CoprimeQ[i, p], {i, t}]

but I don't see how to proceed efficiently from there. Or perhaps there's a better way altogether.

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Try this:

p = {2, 3, 5, 7};
t = {10, 15, 24};
sol = Flatten[
  Table[RandomSample[Select[t, CoprimeQ[p[[i]], #] &], 1], {i, 1, 4}]];

CoprimeQ[p[[#]], sol[[#]]] & /@ Range[4]

(*{True, True, True, True}*)
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  • $\begingroup$ Yes, I guess that's much easier than the complicated scheme I was cooking up. Thanks. $\endgroup$ – rogerl Nov 13 '14 at 0:45

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