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A089452
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a(n) = smallest prime k such that k*(prime(n)-1) + prime(n) is prime.
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3
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2, 2, 2, 2, 2, 5, 3, 2, 3, 5, 2, 5, 2, 2, 2, 3, 2, 2, 2, 5, 3, 113, 3, 5, 3, 2, 29, 3, 2, 2, 3, 2, 5, 3, 3, 5, 2, 2, 5, 5, 2, 2, 2, 17, 11, 2, 7, 11, 19, 3, 3, 13, 2, 2, 2, 5, 2, 2, 11, 3, 2, 2, 5, 2, 11, 2, 2, 2, 5, 3, 3, 19, 2, 5, 5, 3, 5, 2, 19, 29, 5, 2
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OFFSET
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2,1
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COMMENTS
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Does every prime appear in this sequence? - Gabriel Cunningham (gcasey(AT)mit.edu), Mar 27 2004
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LINKS
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EXAMPLE
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a(2)=2 because 2*(prime(2)-1) + prime(2) = 7, which is prime.
a(7)=5 because 2*(prime(7)-1) + prime(7) = 49 and 3*(prime(7)-1) + prime(7) = 65, both of which are composite, but 5*(prime(7)-1) + prime(7) = 97, which is prime.
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MATHEMATICA
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spk[n_]:=Module[{k=2}, While[!PrimeQ[k(n-1)+n], k=NextPrime[k]]; k]; spk/@Prime[Range[2, 110]] (* Harvey P. Dale, Nov 06 2014 *)
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PROG
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(PARI) a(n) = p = prime(n); forprime(k=2, , if (isprime(k*(p-1) + p), return(k); )); \\ Michel Marcus, Nov 18 2014
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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More terms from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 27 2004
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STATUS
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approved
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