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A005538 Numbers n such that 8*3^n + 1 is prime.
(Formerly M1729)
2
2, 7, 8, 10, 22, 52, 58, 76, 130, 143, 331, 332, 980, 1282, 1655, 4031, 4600, 18880, 89482, 130592, 323860, 872171, 1172480 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

From R. J. Mathar, Mar 01 2010: (Start)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

a(21) > 2*10^5. - Robert Price, Mar 16 2014

2*A056799(16) = 2*161930 = 323860 is also a member, but I do not know if it is the next member. - Jeppe Stig Nielsen, Oct 13 2019

a(22) > 8*10^5. - Paul S. Vanderveen, May 02 2020

REFERENCES

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

LINKS

Table of n, a(n) for n=1..23.

Prime Wiki, General Proth prime of the form 8*3^n+1

H. C. Williams and C. R. Zarnke, Some prime numbers of the forms 2*3^n+1 and 2*3^n-1, Math. Comp., 26 (1972), 995-998.

MATHEMATICA

Select[Range[5000], PrimeQ[8*3^# + 1] &] (* Michael De Vlieger, Jun 26 2017 *)

PROG

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

CROSSREFS

Cf. A056799.

Sequence in context: A075766 A258823 A226612 * A329648 A206331 A053013

Adjacent sequences:  A005535 A005536 A005537 * A005539 A005540 A005541

KEYWORD

nonn,hard,changed

AUTHOR

N. J. A. Sloane

EXTENSIONS

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

a(19)-a(20) from Robert Price, Mar 16 2014

a(21) from Paul S. Vanderveen, Feb 09 2020

a(22) from Paul S. Vanderveen, Jun 12 2020

a(23) (found May 17 2020) from Paul S. Vanderveen, Jul 04 2020

STATUS

approved

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Last modified July 15 15:15 EDT 2020. Contains 335774 sequences. (Running on oeis4.)