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A086005
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Semiprimes sandwiched between semiprimes.
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11
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34, 86, 94, 122, 142, 202, 214, 218, 302, 394, 446, 634, 698, 842, 922, 1042, 1138, 1262, 1346, 1402, 1642, 1762, 1838, 1894, 1942, 1982, 2102, 2182, 2218, 2306, 2362, 2434, 2462, 2518, 2642, 2722, 2734, 3098, 3386, 3602, 3694, 3866, 3902
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OFFSET
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1,1
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COMMENTS
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a(n) = 2*A086006(n).
These are some of the balanced semiprimes (see A213025). - Alonso del Arte, Jun 04 2012
a(n) = A056809(n)+1. - Zak Seidov, Sep 30 2012
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..10000
Eric Weisstein's World of Mathematics, Semiprime
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EXAMPLE
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94=47*2: 94-1=3*31 and 94+1=5*19, therefore 94 is a term.
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MATHEMATICA
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u[n_]:=Plus@@Last/@FactorInteger[n]==2; lst={}; Do[If[u[n], sp=n; If[u[sp-1]&&u[sp+1], AppendTo[lst, sp]]], {n, 8!}]; lst (* From Vladimir Joseph Stephan Orlovsky, Nov 16 2009 *)
(* First run program for A109611 to define semiPrimeQ *) Select[Range[4000], Union[{semiPrimeQ[# - 1], semiPrimeQ[#], semiPrimeQ[# + 1]}] == {True} &] (* Alonso del Arte, Jun 03 2012 *)
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PROG
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(Haskell)
a086005 n = a086005_list !! (n-1)
a086005_list = f a001358_list where
f (x:sps'@(y:z:sps)) | z == x + 2 = y : f sps'
| otherwise = f sps'
-- Reinhard Zumkeller, Jun 10 2012
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CROSSREFS
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Cf. A001358, A056809.
Sequence in context: A092223 A046764 A213025 * A169834 A140602 A067977
Adjacent sequences: A086002 A086003 A086004 * A086006 A086007 A086008
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KEYWORD
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nonn
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AUTHOR
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Reinhard Zumkeller, Jul 07 2003
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STATUS
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approved
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