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Search: id:A117880
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| A117880 |
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a(1) = 4; a(n) is smallest semiprime > 2*a(n-1). |
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+0
1
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| 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; listen)
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OFFSET
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1,1
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COMMENT
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Comment from Zak Seidov (zakseidov(AT)yahoo.com), Dec 24 2007: 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, ...
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EXAMPLE
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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.
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MATHEMATICA
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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 (zakseidov(AT)yahoo.com), Dec 24 2007
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CROSSREFS
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Semiprime analogue of A055496.
Cf. A001358, A076656.
Sequence in context: A009887 A009914 A048638 this_sequence A027973 A103040 A084861
Adjacent sequences: A117877 A117878 A117879 this_sequence A117881 A117882 A117883
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KEYWORD
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easy,nonn,less
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AUTHOR
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Jonathan Vos Post (jvospost2(AT)yahoo.com), May 04 2006
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EXTENSIONS
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Edited by njas, Jul 01 2008 at the suggestion of R. J. Mathar.
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