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A059242 Numbers n such that 2^n + 5 is prime. 18
1, 3, 5, 11, 47, 53, 141, 143, 191, 273, 341, 16541, 34001, 34763, 42167, 193965, 282203 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The subsequence of primes starts 3, 5, 11, 47, 53, 191, ... - Vincenzo Librandi, Aug 07 2010

For k in this sequence, 2^(k-1)*(2^k+5) is in A141548: numbers of deficiency 6. - M. F. Hasler, Apr 23 2015

a(18) > 5*10^5. - Robert Price, Aug 23 2015

a(18) > 6*10^5. - Tyler NeSmith, Jan 18 2021

LINKS

Table of n, a(n) for n=1..17.

Keith Conrad, Square patterns and infinitude of primes, University of Connecticut, 2019.

Henri Lifchitz and Renaud Lifchitz (Editors), PRP Top Records, of the form 2^n+5.

EXAMPLE

2^3 + 5 = 13 is prime but 2^4 + 5 = 21 is not.

MATHEMATICA

Select[Range[20000], PrimeQ[2^#+5]&] (* Vladimir Joseph Stephan Orlovsky, Feb 26 2011 *)

PROG

(PARI) is(n)=ispseudoprime(2^n+5) \\ M. F. Hasler, Apr 23 2015

CROSSREFS

Sequence in context: A332968 A144467 A049883 * A175173 A268510 A004203

Adjacent sequences: A059239 A059240 A059241 * A059243 A059244 A059245

KEYWORD

nonn,hard,more

AUTHOR

Tony Davie (ad(AT)dcs.st-and.ac.uk), Jan 21 2001

EXTENSIONS

More terms from Santi Spadaro, Oct 04 2002

a(12) from Hans Havermann, Oct 07 2002

a(13)-a(15) from Charles R Greathouse IV, Oct 07 2011

a(16)-a(17) from Robert Price, Dec 06 2013

STATUS

approved

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Last modified January 30 07:12 EST 2023. Contains 359939 sequences. (Running on oeis4.)