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A096820 Exponents k such that 2^k - 21 is prime. 14
5, 6, 7, 9, 11, 13, 14, 21, 23, 41, 46, 89, 110, 389, 413, 489, 869, 1589, 1713, 2831, 10843, 11257, 16949, 24513, 39621, 43349, 62941, 96094, 139237, 145289, 264683, 396790 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Similar to A057202 (which allows negative primes): this sequence is obtained by dropping the first four terms of A057202. - Joerg Arndt, Oct 05 2012

LINKS

Table of n, a(n) for n=1..32.

Henri Lifchitz and Renaud Lifchitz, Search for 2^n-21, PRP Top Records

EXAMPLE

k=5: 32 - 21 = 11 is prime;

k=7: 128 - 21 = 107 is prime.

MATHEMATICA

Select[Range[5, 20000], PrimeQ[2^#-21]&] (* Vladimir Joseph Stephan Orlovsky, Feb 27 2011 *)

PROG

(Sage)

def is_A096820(n):

t = 2^n-21

return t > 1 and is_prime(t)

def A096820_list(up_to):

return [n for n in range(up_to) if is_A096820(n)]

A096820_list(100) # Peter Luschny, Oct 04 2012

CROSSREFS

Exponents for primes of 2^n-d form: A000043 (d=1), A050414 (d=3), A059608 (d=5), A059609 (d=7), A059610 (d=9), A096817 (d=11), A096818 (d=13), A059612 (d=15), A059611 (d=17), A096819 (d=19), A096820 (d=21).

Cf. A096502

Sequence in context: A132829 A180061 A196029 * A175154 A355239 A030388

Adjacent sequences: A096817 A096818 A096819 * A096821 A096822 A096823

KEYWORD

nonn,more

AUTHOR

Labos Elemer, Jul 13 2004

EXTENSIONS

a(23)-a(24) from Max Alekseyev, a(25) from Donovan Johnson, a(26)-a(28) from Henri Lifchitz, a(29)-a(30) from Lelio R Paula, added by Max Alekseyev, Feb 10 2012

a(31)-a(32) from Lelio R Paula, added by Max Alekseyev, Oct 24 2013

STATUS

approved

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Last modified December 6 10:03 EST 2022. Contains 358630 sequences. (Running on oeis4.)