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A117405 Semiprime nearest to 2^n. (In case of a tie, choose the smaller). 5

%I

%S 4,4,4,9,14,33,65,129,254,511,1027,2047,4097,8193,16382,32765,65531,

%T 131073,262142,524289,1048577

%N Semiprime nearest to 2^n. (In case of a tie, choose the smaller).

%C Semiprime analog of A117387 Prime nearest to 2^n. (In case of a tie, choose the smaller). After n=2, never again is a(n) a power of 2.

%F a(n) = 2^n + A117406(n).

%e a(0) = 4 because 2^0 + 3 = 4 = A001358(1) and no semiprime is closer to 2^0.

%e a(1) = 4 because 2^1 + 2 = 4 = A001358(1) and no semiprime is closer to 2^1.

%e a(2) = 4 because 2^2 + 0 = 4 = A001358(1) and no semiprime is closer to 2^2.

%e a(3) = 9 because 2^3 + 1 = 9 = 3^2 = A001358(3), no semiprime is closer to 2^3.

%e a(4) = 14 because 2^4 - 2 = 14 = 2 * 7 = A001358(5), no semiprime is closer.

%e a(5) = 33 because 2^5 + 1 = 33 = 3 * 11 and no semiprime is closer to 2^5.

%e a(6) = 65 because 2^6 + 1 = 65 = 5 * 13 and no semiprime is closer to 2^6.

%e a(7) = 129 because 2^7 + 1 = 129 = 3 * 43 and no semiprime is closer to 2^7.

%e a(8) = 254 because 2^8 - 2 = 254 = 2 * 127 and no semiprime is closer to 2^8.

%Y Cf. A000079, A001358, A117387, A117406.

%K easy,nonn,less

%O 0,1

%A _Jonathan Vos Post_, Mar 13 2006

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Last modified August 28 03:16 EDT 2015. Contains 261112 sequences.