cp's OEIS Frontend

A096820 Numbers k such that 2^k - 21 is prime.

%I A096820 #33 Nov 25 2023 08:42:34
%S A096820 5,6,7,9,11,13,14,21,23,41,46,89,110,389,413,489,869,1589,1713,2831,
%T A096820 10843,11257,16949,24513,39621,43349,62941,96094,139237,145289,264683,
%U A096820 396790,420694,439931,659589,783893,840203,944561
%N A096820 Numbers k such that 2^k - 21 is prime.
%C A096820 Similar to A057202 (which allows negative primes): this sequence is obtained by dropping the first four terms of A057202. - _Joerg Arndt_, Oct 05 2012
%H A096820 Henri Lifchitz and Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=2%5En-21">Search for 2^n-21</a>, PRP Top Records.
%e A096820 k = 5: 32 - 21 = 11 is prime.
%e A096820 k = 7: 128 - 21 = 107 is prime.
%t A096820 Select[Range[5,20000],PrimeQ[2^#-21]&] (* _Vladimir Joseph Stephan Orlovsky_, Feb 27 2011 *)
%o A096820 (Sage)
%o A096820 def is_A096820(n):
%o A096820     t = 2^n-21
%o A096820     return t > 1 and is_prime(t)
%o A096820 def A096820_list(up_to):
%o A096820     return [n for n in range(up_to) if is_A096820(n)]
%o A096820 A096820_list(100)  # _Peter Luschny_, Oct 04 2012
%Y A096820 Cf. A096502.
%Y A096820 Cf. Sequences of numbers k such that 2^k - d is prime: 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), this sequence (d=21), A057220 (d=23), A356826 (d=29).
%K A096820 nonn,more
%O A096820 1,1
%A A096820 _Labos Elemer_, Jul 13 2004
%E A096820 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
%E A096820 a(31)-a(32) from Lelio R Paula, added by _Max Alekseyev_, Oct 24 2013
%E A096820 a(33)-a(34) found by Lelio R Paula, a(35)-a(38) found by Stefano Morozzi, added by _Elmo R. Oliveira_, Nov 24 2023