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A135507
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a(1)=1; for n>1, a(n)=2*a(n-1)+lcm(a(n-1),n).
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0
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1, 4, 20, 60, 180, 540, 4860, 19440, 58320, 174960, 2274480, 6823440, 20470320, 184232880, 552698640, 1658095920, 31503822480, 94511467440, 283534402320, 850603206960, 7655428862640, 99520575214320, 2488014380358000
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| This sequence has fascinating properties related to primes and especially to twin primes. For instance a(n+1)/a(n)-2 is never the largest prime of a twin pair (except for 7).
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REFERENCES
| Benoit Cloitre, Beyond Rowland's gcd sequence, in preparation, 2008
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PROG
| (PARI) x1=1; for(n=2, 40, x2=2*x1+lcm(x1, n); t=x1; x1=x2; print1(x2, ", "))
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CROSSREFS
| Cf. A106108.
Sequence in context: A196680 A033488 A018211 * A197404 A169637 A169638
Adjacent sequences: A135504 A135505 A135506 * A135508 A135509 A135510
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KEYWORD
| nonn,nice
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AUTHOR
| Benoit Cloitre (benoit7848c(AT)orange.fr), Feb 09 2008, Feb 10 2008
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