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A118909
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a(1) = 4; a(n) is least semiprime > a(n-1)^2.
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2
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4, 21, 445, 198026, 39214296677, 1537761063871773242347, 2364709089560047865452947255794201194068433, 5591849078247910476736920566826713466552016538943524658263883555662554776622687075541
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OFFSET
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1,1
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COMMENTS
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Semiprime analogue of A055496 a(1) = 2; a(n) is smallest prime > 2*a(n-1). See also A059785 a(n+1)=prevprime(a(n)^2), with a(1) = 2. With that, of course, there's always a prime between n and 2n, so a(n) < 2^n. The obverse of this is A118908 a(1) = 4; a(n) is greatest semiprime < a(n-1)^2.
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LINKS
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Table of n, a(n) for n=1..8.
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EXAMPLE
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a(8) = a(7)^2 + 52 and there is no smaller k such that a(7)^2 + k is semiprime.
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MATHEMATICA
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nxt[n_]:=Module[{sp=n^2+1}, While[PrimeOmega[sp]!=2, sp++]; sp]; NestList[nxt, 4, 7] (* Harvey P. Dale, Oct 22 2012 *)
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CROSSREFS
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Cf. A001358, A055496, A076656, A006992, A005384, A005385, A118908-A118913.
Sequence in context: A198050 A126458 A048164 * A225157 A158947 A000868
Adjacent sequences: A118906 A118907 A118908 * A118910 A118911 A118912
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KEYWORD
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easy,nonn
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AUTHOR
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Jonathan Vos Post, May 05 2006
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STATUS
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approved
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