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The "if" part of the conjecture is true: see the theorems in A309132 and A326690. The values of the numerator when n+1 is prime are A327033. - Jonathan Sondow, Aug 15 2019
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Wikipedia, <a href="https://en.wikipedia.org/wiki/Bernoulli_number">Bernoulli number</a>
Prime(6) = 13 and B(12) = -691/2730, so a(6) = -691/13 + 2730/13^2 = -37.
allocated for Jonathan SondowN(p-1)/p + D(p-1)/p^2 with p the n-th prime and B(k) = N(k)/D(k) the k-th Bernoulli number.
0, 1, 1, 1, 1, -37, -211, 2311, 37153, -818946931, 277930363757, -711223555487930419, -6367871182840222481, 35351107998094669831, 12690449182849194963361, -15116334304443206742413679091, 1431925649981017658678758915153153, -19921854762028779869513196624259348280501
1,6
Prime(6) = 13 and B(12) = -691/2730
a[n_] := With[{p = Prime[n]}, With[{b = BernoulliB[p - 1]}, (p Numerator[b] + Denominator[b])/p^2]];
Table[a[n], {n, 1, 18}]
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Jonathan Sondow, Aug 15 2019
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allocated for Jonathan Sondow
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