A185191 - OEIS

%I #14 Jun 24 2017 19:48:47

%S 2,3,5,3,5,15,15,9,5,19,5,13,15,49,17,9,11,19,5,19,17,21,17,49,27,79,

%T 89,33,41,19,17,25,77,49,17,31,87,19,167,31,17,67,117,69,57,127,65,

%U 111,35,139,143,145,53,67,27,25,57,99,107,31,87,165,83

%N a(n) = 2^n - second largest prime less than 2^n.

%D D. E. Knuth, The Art of Computer Programming Second Edition. Vol. 2, Seminumerical Algorithms. Chapter 4.5.4 Factoring into Primes, Table 1, Page 390, Addison-Wesley, Reading, MA, 1981.

%H G. C. Greubel, <a href="/A185191/b185191.txt">Table of n, a(n) for n = 2..1000</a>

%F a(n) = 2^n - precprime(precprime(2^n)-1)

%e a(2)=2 because precprime(4)=3, and precprime(2)=2.

%t Table[2^n - NextPrime[2^n, -2], {n, 2, 64}] (* _T. D. Noe_, Jan 24 2012 *)

%Y Cf. A013603.

%K nonn

%O 2,1

%A _Washington Bomfim_, Jan 23 2012