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A117405
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Semiprime nearest to 2^n. (In case of a tie, choose the smaller).
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5
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4, 4, 4, 9, 14, 33, 65, 129, 254, 511, 1027, 2047, 4097, 8193, 16382, 32765, 65531, 131073, 262142, 524289, 1048577
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,1
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COMMENTS
| Semiprime analogue 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.
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FORMULA
| a(n) = 2^n + A117406(n).
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EXAMPLE
| a(0) = 4 because 2^0 + 3 = 4 = A001358(1) and no semiprime is closer to 2^0.
a(1) = 4 because 2^1 + 2 = 4 = A001358(1) and no semiprime is closer to 2^1.
a(2) = 4 because 2^2 + 0 = 4 = A001358(1) and no semiprime is closer to 2^2.
a(3) = 9 because 2^3 + 1 = 9 = 3^2 = A001358(3), no semiprime is closer to 2^3.
a(4) = 14 because 2^4 - 2 = 14 = 2 * 7 = A001358(5), no semiprime is closer.
a(5) = 33 because 2^5 + 1 = 33 = 3 * 11 and no semiprime is closer to 2^5.
a(6) = 65 because 2^6 + 1 = 65 = 5 * 13 and no semiprime is closer to 2^6.
a(7) = 129 because 2^7 + 1 = 129 = 3 * 43 and no semiprime is closer to 2^7.
a(8) = 254 because 2^8 - 2 = 254 = 2 * 127 and no semiprime is closer to 2^8.
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CROSSREFS
| Cf. A000079, A001358, A117387, A117406.
Sequence in context: A200600 A048761 A075561 * A013601 A190717 A035618
Adjacent sequences: A117402 A117403 A117404 * A117406 A117407 A117408
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KEYWORD
| easy,nonn,less
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Mar 13 2006
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