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A106821
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Greatest k < n such that 2^n - 2^k -1 is prime or 0 if no such prime exists.
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5
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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LINKS
| Pierre CAMI, Table of n, a(n) for n = 1..10000
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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
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PROG
| (Pari)
for(n=1, 1000, q=1;
forstep(k=n-1, 0, -1,
if(isprime(2^n-2^k-1),
q=0;
print1(k, ", ");
break()
)
);
if(q, print1(0, ", "))
) /* Joerg Arndt, Jan 14 2011 */
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CROSSREFS
| Sequence in context: A128744 A118963 A127641 * A065863 A049272 A181483
Adjacent sequences: A106818 A106819 A106820 * A106822 A106823 A106824
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KEYWORD
| nonn
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AUTHOR
| Pierre CAMI (pierre-cami(AT)bbox.fr), May 18 2005
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