A191013 Largest k such that (2^n-k)*2^n-k is a prime number.

1, 5, 9, 29, 61, 119, 249, 507, 1017, 2045, 4047, 8181, 16381, 32765, 65521, 131069, 262137, 524277, 1048567, 2097145, 4194285, 8388597, 16777185, 33554421, 67108821, 134217725, 268435405, 536870909, 1073741811, 2147483619, 4294967275, 8589934569, 17179869177

Offset

2,2

Links

Table of n, a(n) for n=2..34.

Example

a(2)=1 because (2^2-1)*2^2-1 = 11 is prime,
a(3)=5 because (2^3-5)*2^3-5 = 19 is prime,
a(4)=7 because (2^4-7)*2^4-7 = 137 is prime.

Prog

(PARI) a(n)=forstep(k=2^n-1, 1, -1, ispseudoprime((2^n-k)*2^n-k) & return(k)) \\ M. F. Hasler, Jun 17 2011

Crossrefs

Cf. A000040.

Sequence in context: A303676 A163779 A280487 * A193487 A006131 A352008

Adjacent sequences: A191010 A191011 A191012 * A191014 A191015 A191016

Keyword

nonn

Author

Juri-Stepan Gerasimov, Jun 16 2011

Status

approved