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 A189657 Start with n, apply k->2k+1 until a semiprime is reached; sequence gives the semiprimes. 0
 15, 95, 15, 9, 95, 55, 15, 35, 39, 21, 95, 25, 55, 119, 511, 33, 35, 303, 39, 335, 87, 91, 95, 49, 51, 215, 55, 57, 119, 123, 511, 65, 543, 69, 143, 295, 303, 77, 159, 327, 335, 85, 87, 5759, 91, 93, 95, 391, 799, 203, 415, 54271, 215, 219, 111, 3647, 115 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS This is to semiprimes A001358 as A051919 is to primes A000040. Is this sequence defined for all n? LINKS EXAMPLE a(0) = 15 in 4 steps because 2*(2*(2*((2*0)+1)+1)+1)+1 = 15 = 3*5 is semiprime. a(1) = 15 in 3 steps because 2*(2*((2*1) + 1)+1)+1 = 15 = 3*5 a(2) = 95 in 5 steps because 2*(2*(2*(2*(2*2 + 1)+1)+1)+1)+1 = 95 = 5*19. MATHEMATICA semiPrimeQ[n_] := Total[FactorInteger[n]][[2]]==2; Table[k = n; While[k = 2 k + 1; ! semiPrimeQ[k]]; k, {n, 100}] (* T. D. Noe, Apr 29 2011 *) CROSSREFS Cf. A001358, A051919. Sequence in context: A126483 A226766 A114240 * A190274 A052459 A044266 Adjacent sequences:  A189654 A189655 A189656 * A189658 A189659 A189660 KEYWORD nonn,easy AUTHOR Jonathan Vos Post, Apr 25 2011 EXTENSIONS Extended by T. D. Noe, Apr 29 2011 STATUS approved

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Last modified January 29 10:34 EST 2020. Contains 331337 sequences. (Running on oeis4.)