A191013 - OEIS

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

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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