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%I A247307 #21 Apr 08 2021 05:36:10
%S A247307 0,6,20,63,204,682,2340,381300,1398101,5162220,71582788,1010580540,
%T A247307 14467258260,3059510616420,2573485501354569,9938978487990060,
%U A247307 148764065110560900,510526106256177860940,117943982401427236556700,1799331452449680632120820
%N A247307 Numbers of the form (4^k - 4)/k.
%C A247307 Subsequence of A246445.
%C A247307 Generated by k = 1, 2, 3, 4, 5, 6, 7, 11, 12, 13, 15, 17, 19, 23, 28, 29, 31,. ..
%C A247307 This set of k contains all terms of A122781 and all primes. [It contains the primes because j^p == j (mod p) for every integer j if p is prime; see e.g. the corollary 4.4 to the Lagrange theorem in Jones et al.]
%H A247307 G. A. Jones and J. M. Jones, <a href="http://dx.doi.org/10.1007/978-1-4471-0613-5_4">Congruences with a prime-power modulus</a>, p 65-82 in "Elementary Number Theory", Springer Undergraduate Mathematics Series, (1988).
%e A247307 a(9) = 1398101 because (4^12 - 4)/12 = 1398101 for k = 12.
%o A247307 (PARI) lista(nn) = {for (k=1, nn, va = (4^k - 4)/k; if (type(va) == "t_INT", print1(va, ", ")););} \\ _Michel Marcus_, Sep 12 2014
%Y A247307 Cf. A020136, A064535, A122781, A246445, A247033.
%K A247307 nonn
%O A247307 1,2
%A A247307 _Juri-Stepan Gerasimov_, Sep 11 2014

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