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 A111010 Primes of the form (3^k - (-1)^k)/4. 3
 2, 7, 61, 547, 398581, 23535794707, 82064241848634269407 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The next term is too large to include. Is there an infinity of primes in this sequence? All a(n), except a(1) = 2, are primes of the form (3^k + 1)/4. Corresponding numbers k such that (3^k + 1)/4 is prime are listed in A007658(n) = {3, 5, 7, 13, 23, 43, 281, 359, 487, 577, ...}. All such numbers k are primes. a(1) = 2 is the only prime of the form (3^k - 1)/4. - Alexander Adamchuk, Nov 19 2006 REFERENCES Prime Obsession, John Derbyshire, Joseph Henry Press, April 2004, p 16. LINKS Alexander Adamchuk, Nov 19 2006, Table of n, a(n) for n = 1..11 FORMULA Given a(0)=1, b(0)=1, then for i=1, 2, ..., a(i)/b(i) = (a(i-1) + 2*b(i-1)) /(a(i-1) + b(i-1)). MATHEMATICA Do[f=(3^n - (-1)^n)/4; If[PrimeQ[f], Print[{n, f}]], {n, 1, 577}] (* Alexander Adamchuk, Nov 19 2006 *) PROG (PARI) primenum(n, k, typ) = \ k=mult, typ=1 num, 2 denom. ouyput prime num or denom. { local(a, b, x, tmp, v); a=1; b=1; for(x=1, n, tmp=b; b=a+b; a=k*tmp+a; if(typ==1, v=a, v=b); if(isprime(v), print1(v", "); ) ); print(); print(a/b+.) } CROSSREFS Cf. A007658, A015518. Sequence in context: A065397 A255904 A046846 * A089307 A102896 A088107 Adjacent sequences:  A111007 A111008 A111009 * A111011 A111012 A111013 KEYWORD nonn AUTHOR Cino Hilliard, Oct 02 2005 EXTENSIONS Edited by Alexander Adamchuk, Nov 19 2006 STATUS approved

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Last modified August 18 01:04 EDT 2019. Contains 326059 sequences. (Running on oeis4.)