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Search: id:A140135
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| A140135 |
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Product of largest semiprime <= n and smallest semiprime >= n. |
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+0
1
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| 16, 24, 36, 54, 54, 81, 100, 140, 140, 140, 196, 225, 315, 315, 315, 315, 315, 441, 484, 550, 550, 625, 676, 858, 858, 858, 858, 858, 858, 1089, 1156, 1225, 1330, 1330, 1444, 1521, 1794, 1794, 1794, 1794, 1794, 1794, 2116, 2254, 2254, 2401, 2499, 2601, 2805
(list; graph; listen)
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OFFSET
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4,1
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COMMENT
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This is to A030664 as semiprimes A001358 are to primes A000040. Subset of A014613.
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FORMULA
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a(n) = MAX{j in A001358 and j <= n} * MIN{j in A001358 and j >= n}
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EXAMPLE
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a(10) = 100 because the largest semiprime <= 10 is 10, the smallest semiprime >= 10 is 10 and 10*10=100.
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MAPLE
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isA001358 := proc(n) RETURN( numtheory[bigomega](n) = 2) ; end: A001358 := proc(n) option remember ; local a; if n = 1 then 4; else for a from A001358(n-1)+1 do if isA001358(a) then RETURN(a) ; fi ; od: fi ; end: prevsemiprime := proc(n) local a; for a from n to 4 by -1 do if isA001358(a) then RETURN(a) ; fi ; od: RETURN(-1) ; end: nextsemiprime := proc(n) local a; for a from n do if isA001358(a) then RETURN(a) ; fi ; od: RETURN(-1) ; end: A140135 := proc(n) prevsemiprime(n)*nextsemiprime(n) ; end: seq(A140135(n), n=4..80) ; - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), May 11 2008
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CROSSREFS
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Cf. A001358, A014613, A030664.
Sequence in context: A014613 A046370 A103248 this_sequence A120142 A110228 A045875
Adjacent sequences: A140132 A140133 A140134 this_sequence A140136 A140137 A140138
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KEYWORD
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easy,nonn
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AUTHOR
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Jonathan Vos Post (jvospost3(AT)gmail.com), May 09 2008
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EXTENSIONS
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More terms from R. J. Mathar (mathar(AT)strw.leidenuniv.nl), May 11 2008
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