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 A191013 Largest k such that (2^n-k)*2^n-k is a prime number. 0

%I

%S 1,5,9,29,61,119,249,507,1017,2045,4047,8181,16381,32765,65521,131069,

%T 262137,524277,1048567,2097145,4194285,8388597,16777185,33554421,

%U 67108821,134217725,268435405,536870909,1073741811,2147483619,4294967275,8589934569,17179869177

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

%e a(2)=1 because (2^2-1)*2^2-1 = 11 is prime,

%e a(3)=5 because (2^3-5)*2^3-5 = 19 is prime,

%e a(4)=7 because (2^4-7)*2^4-7 = 137 is prime.

%o (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

%Y Cf. A000040.

%K nonn

%O 2,2

%A _Juri-Stepan Gerasimov_, Jun 16 2011

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Last modified March 25 03:37 EDT 2023. Contains 361511 sequences. (Running on oeis4.)