

A106821


Greatest k < n such that 2^n  2^k 1 is prime or 0 if no such prime exists.


5



0, 0, 2, 3, 3, 5, 0, 7, 7, 5, 3, 9, 11, 13, 0, 11, 11, 17, 11, 19, 17, 16, 0, 21, 13, 9, 0, 23, 17, 18, 0, 31, 23, 28, 31, 34, 0, 33, 0, 38, 31, 37, 0, 30, 43, 17, 31, 45, 41, 37, 7, 47, 41, 44, 0, 42, 55, 0, 27, 42, 43, 61, 0, 29
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OFFSET

1,3


LINKS

Pierre CAMI, Table of n, a(n) for n = 1..10000


EXAMPLE

2^3  2^2 1 = 3 prime so a(3)=2
2^4  2^3 1 = 7 prime so a(4)=3
2^5  2^3 1 = 23 prime so a(5)=3
2^6  2^5 1 = 31 prime so a(6)=5


MATHEMATICA

kk[n_]:=Module[{k=n1, n2=2^n}, While[!PrimeQ[n22^k1]&&k>0, k]; k]; Array[kk, 70] (* Harvey P. Dale, Mar 09 2012 *)


PROG

(PARI)
for(n=1, 1000, q=1;
forstep(k=n1, 0, 1,
if(isprime(2^n2^k1),
q=0;
print1(k, ", ");
break()
)
);
if(q, print1(0, ", "))
) /* Joerg Arndt, Jan 14 2011 */


CROSSREFS

Sequence in context: A118963 A127641 A328730 * A065863 A049272 A181483
Adjacent sequences: A106818 A106819 A106820 * A106822 A106823 A106824


KEYWORD

nonn


AUTHOR

Pierre CAMI, May 18 2005


EXTENSIONS

Extra numbers to right of data removed from bfile by Andrew Howroyd, Feb 23 2018


STATUS

approved



