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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

Table of n, a(n) for n=1..30.

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.)