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 A173236 Primes of the form 2^r * 13^s + 1. 3
 2, 3, 5, 17, 53, 257, 677, 3329, 13313, 35153, 65537, 2768897, 13631489, 2303721473, 3489660929, 4942652417, 11341398017, 10859007357953, 1594691292233729, 31403151600910337, 310144109150467073, 578220423796228097 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Necessarily r is even (elementary proof by induction). s=0 is (trivial) case of 2 and the known five Fermat primes: 2, 3, 5, 17, 257, 65537 (A092506). Fermat prime exponents r are 0, 1, 2, 4, 8, 16. REFERENCES Emil Artin: Galoissche Theorie, Verlag Harri Deutsch, Zuerich, 1973 Leonard E. Dickson: History of the Theory of numbers, vol. I, Dover Publications, 2005 Paulo Ribenboim, Wilfrid Keller, Joerg Richstein: Die Welt der Primzahlen, Springer-Verlag GmbH Berlin, 2006 LINKS EXAMPLE 2^0*13^0 + 1 = 2 = prime(1) => a(1). 2^10*13^1 + 1 = 13313 = prime(1581) => a(9). list of (r,s): (0,0), (1,0), (2,0), (4,0), (2,1), (8,0), (2,2), (8,1), (10,1), (4,3), (16,0), (14,2), (20,1), (20,3), (28,1), (10,6), (26,2), (10,9), (32,5), (40,4), (10,13), (22,10), (32,8), (48,4), (20,13), (2,18), (28,11), (50,6). PROG (GAP) K:=10^7;; # to get all terms <= K. A:=Filtered([1..K], IsPrime);; B:=List(A, i->Factors(i-1));; C:=[];;  for i in B do if Elements(i)=[2] or Elements(i)=[2, 13] then Add(C, Position(B, i)); fi; od; A173236:=Concatenation([2], List(C, i->A[i])); # Muniru A Asiru, Sep 10 2017 CROSSREFS Cf. A092506, A005105, A092506, A173062. Sequence in context: A235636 A268130 A103074 * A268209 A245641 A082979 Adjacent sequences:  A173233 A173234 A173235 * A173237 A173238 A173239 KEYWORD nonn AUTHOR Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Feb 13 2010 STATUS approved

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Last modified March 22 01:06 EDT 2019. Contains 321406 sequences. (Running on oeis4.)