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 A273948 Odd prime factors of generalized Fermat numbers of the form 7^(2^m) + 1 with m >= 0. 8
 5, 17, 257, 353, 769, 1201, 12289, 13313, 35969, 65537, 114689, 163841, 169553, 7699649, 9379841, 11886593, 28667393, 64749569, 70254593, 134818753, 197231873, 4643094529, 19847446529, 47072139617, 206158430209, 452850614273, 531968664833, 943558259713 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Odd primes p other than 3 such that the multiplicative order of 7 (mod p) is a power of 2. From Robert Israel, Jun 16 2016: (Start) If p is in the sequence, then for each m either p | 7^(2^k)+1 for some k < m or 2^m | p-1.  Thus all members except 5, 17, 353, 1201, 169553, 7699649, 134818753, 47072139617 are congruent to 1 mod 2^7. The intersection of this sequence and A019337 is A019434 minus {3}. (End) REFERENCES Hans Riesel, Common prime factors of the numbers A_n=a^(2^n)+1, BIT 9 (1969), pp. 264-269. LINKS Arkadiusz Wesolowski, Table of n, a(n) for n = 1..34 Anders Björn and Hans Riesel, Factors of generalized Fermat numbers, Math. Comp. 67 (1998), no. 221, pp. 441-446. Anders Björn and Hans Riesel, Table errata to “Factors of generalized Fermat numbers”, Math. Comp. 74 (2005), no. 252, p. 2099. Anders Björn and Hans Riesel, Table errata 2 to "Factors of generalized Fermat numbers", Math. Comp. 80 (2011), pp. 1865-1866. Harvey Dubner and Wilfrid Keller, Factors of Generalized Fermat Numbers, Math. Comp. 64 (1995), no. 209, pp. 397-405. OEIS Wiki, Generalized Fermat numbers MAPLE filter:= proc(t)   if not isprime(t) then return false fi;   7 &^ (2^padic:-ordp(t-1, 2)) mod t = 1 end proc: select(filter, [seq(i, i=5..10^6, 2)]); # Robert Israel, Jun 16 2016 MATHEMATICA Select[Prime@Range[3, 10^5], IntegerQ@Log[2, MultiplicativeOrder[7, #]] &] CROSSREFS Cf. A023394, A072982, A078304, A273945 (base 3), A273946 (base 5), A273947 (base 6), A273949 (base 11), A273950 (base 12). Sequence in context: A077718 A235461 A271660 * A271657 A273999 A222008 Adjacent sequences:  A273945 A273946 A273947 * A273949 A273950 A273951 KEYWORD nonn AUTHOR Arkadiusz Wesolowski, Jun 05 2016 STATUS approved

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Last modified August 10 11:52 EDT 2020. Contains 336379 sequences. (Running on oeis4.)