A079363 Primes of the form 2*3^k - 1.

5, 17, 53, 4373, 13121, 1062881, 6973568801, 188286357653, 15251194969973, 100063090197999413, 1046695266054721074427023041, 763040848953891663257299797617, 556256778887387022514571552463521

Offset: 1

Author

Cino Hilliard, Feb 15 2003

Keywords

• nonn

Comments

Sum of reciprocals = 0.2779972845973183835923785945..

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..20

Crossrefs

Cf. A003306 (n such that 2*3^n+1 is prime), A003307 (n such that 2*3^n-1 is prime).

Programs

Magma

[a: n in [1..200] | IsPrime(a) where a is  2*3^n-1 ]; // Vincenzo Librandi, Dec 09 2011

Maple

select(isprime,[2*3^k-1$k=0..200]); # Muniru A Asiru, Nov 24 2018

Mathematica

Select[2*3^Range[100]-1, PrimeQ]

PARI

\\ Primes in the sequence of sums of alternating powers of 3
pseq3(n) = { j=a=1; p=1; sr=0; while(j<=n, a = a + 3^(p); if(isprime(a),print1(a", "); sr+=1.0/a; ); a = a+3^(p-1); if(isprime(a),print1(a", "); sr+=1.0/a; ); p+=1; j+=2; ); print(); print(sr); }