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 A117880 a(1) = 4; a(n) is smallest semiprime > 2*a(n-1). 0
 4, 9, 21, 46, 93, 187, 377, 755, 1513, 3027, 6059, 12127, 24257, 48529, 97059, 194127, 388257, 776515, 1553033, 3106083, 6212177, 12424355, 24848723, 49697447, 99394909, 198789819, 397579639, 795159283, 1590318573, 3180637153 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(1)=4, a(n)=2*a(n-1)+k, where k is least positive integer chosen so that a(n) is the product of two primes. Corresponding k's are 1, 3, 4, 1, 1, 3, 1, 3, 1, 5, 9, 3, 15, 1, 9, 3, 1, 3, 17, 11, 1, 13, 1, 15, 1, 1, 5, 7, 7, 11, 5, 5, 15, 1, 3, 9, 9, 5, 7, 8, ... - Zak Seidov, Dec 24 2007 LINKS EXAMPLE a(1)=4, then k=1, a(2)=2*4+1=9, k=3, a(3)=2*9+3=21, k=4, a(4)=2*21+4=46, k=1, a(5)=2*46+1=93, k=1, a(6)=2*93+1=187. MATHEMATICA a=4; Do[Do[b=2a+n; If[2==Plus@@FactorInteger[b][[All, 2]], Print[{b, n}]; Break[]], {n, 1000}]; a=b, {40}] - Zak Seidov, Dec 24 2007 CROSSREFS Semiprime analog of A055496. Cf. A001358, A076656. Sequence in context: A009914 A048638 A144527 * A027973 A103040 A084861 Adjacent sequences:  A117877 A117878 A117879 * A117881 A117882 A117883 KEYWORD easy,nonn,less AUTHOR Jonathan Vos Post, May 04 2006 EXTENSIONS Edited by N. J. A. Sloane, Jul 01 2008 at the suggestion of R. J. Mathar STATUS approved

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Last modified January 19 20:11 EST 2019. Contains 319309 sequences. (Running on oeis4.)