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 A195418 a(n) = phi(C(n)) / gcd(C(n)-1, phi(C(n)), where C(n) is the n-th Cullen number. 1
 1, 1, 3, 5, 3, 33, 5, 33, 341, 1045, 189, 1299, 891, 4437, 9477, 581, 3855, 105525, 27825, 23751, 173043, 10531345, 56511, 2386125, 380955, 256861, 24926139, 5108467, 32397379, 930343095, 930291, 36512775 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS When C(n) is prime (or 1), then a(n) = 1; that is, n is in A005849. On the penultimate page of their paper, Grau and Luca ask for "a good (large) lower bound on this quantity which is valid for all n and which tends to infinity with n." LINKS Amiram Eldar, Table of n, a(n) for n = 0..848 José María Grau Ribas and Florian Luca, Cullen numbers with the Lehmer property, Proceedings of the American Mathematical Society, Vol. 140, No. 1 (2012), pp. 129-134, preprint, arXiv:1103.3578 [math.NT], Mar 18, 2011. EXAMPLE a(2) = 3 because the second Cullen number is 9; phi(9) = 6, therefore 6/gcd(8, 6) = 6/2 = 3. MATHEMATICA cullen[n_] := n(2^n) + 1; Table[EulerPhi[cullen[n]]/GCD[cullen[n] - 1, EulerPhi[cullen[n]]], {n, 0, 39}] PROG (PARI) a(n)=my(C=n<

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Last modified January 20 18:38 EST 2022. Contains 350472 sequences. (Running on oeis4.)