

A117880


a(1) = 4; a(n) is smallest semiprime > 2*a(n1).


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
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OFFSET

1,1


COMMENTS

a(1)=4, a(n)=2*a(n1)+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 A348407 A103040
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



