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A005538 Numbers n such that 8*3^n + 1 is prime.
(Formerly M1729)
1

%I M1729

%S 2,7,8,10,22,52,58,76,130,143,331,332,980,1282,1655,4031,4600,18880,

%T 89482,130592

%N Numbers n such that 8*3^n + 1 is prime.

%C Contribution from _R. J. Mathar_, Mar 01 2010: (Start)

%C The row r=4 in the table of "Numbers n such that 2*r*3^n+1 is prime":

%C .1,..2,..4,..5,..6,..9,.16,.17,.30,.54,.57,.60,.65,.132,.180,

%C .1,..2,..3,..6,.14,.15,.39,201,249,

%C .1,..3,..4,..5,..8,.15,.16,.29,.53,.56,.59,.64,131,179,

%C .2,..7,..8,.10,.22,.52,.58,.76,130,143,

%C .1,..3,..4,..7,..9,.12,.18,.22,102,112,157,162,289,

%C .1,..2,..5,.13,.14,.38,200,248,

%C .1,..2,..3,.18,.22,.26,.27,.33,.39,.57,.62,.94,145,246,

%C .3,..4,..5,.12,.24,.36,.77,195,296,297,

%C .2,..3,..4,..7,.14,.15,.28,.52,.55,.58,.63,130,178,

%C .1,..2,..3,..4,..5,..8,.16,.19,.28,.50,134,280,

%C .1,..2,..4,..5,.10,.12,.14,.24,.34,.37,.52,.56,.65,.68,.96,106,128,156,169,236,254,

%C .1,..6,..7,..9,.21,.51,.57,.75,129,142,

%C .1,.12,.15,.17,.20,.29,.31,.32,.35,.37,.77,.95,193,203,224,296,

%C .3,..4,..8,.11,.14,.15,.18, (End)

%C a(21) > 2*10^5. - _Robert Price_, Mar 16 2014

%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

%H H. C. Williams and C. R. Zarnke, <a href="http://www.jstor.org/stable/2005886">Some prime numbers of the forms 2*3^n+1 and 2*3^n-1</a>, Math. Comp., 26 (1972), 995-998.

%t Select[Range[5000], PrimeQ[8*3^# + 1] &] (* _Michael De Vlieger_, Jun 26 2017 *)

%o (PARI) v=[ ]; for(n=0,2000, if(isprime(8*3^n+1),v=concat(v,n),)); v

%K nonn,hard

%O 1,1

%A _N. J. A. Sloane_

%E More terms from Douglas Burke (dburke(AT)nevada.edu)

%E a(19)-a(20) from _Robert Price_, Mar 16 2014

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Last modified May 19 02:45 EDT 2019. Contains 323377 sequences. (Running on oeis4.)